Apparatus and method for testing circuit board

ABSTRACT

An apparatus for testing a circuit board has a holder for holding the circuit board, a detector for detecting the electrical characteristics of the circuit board, and a controller for controlling laser plasma switches. The detector has a path forming unit positioned away from the circuit board by a predetermined gap. The path forming unit forms a first conductive path between a position corresponding to a first test pad on a trace of the circuit board and a first power source, as well as a second conductive path between a position corresponding to a second test pad on another trace of the circuit board and a second power source. The controller emits a laser beam to a first space between the first test pad and the first conductive path and another laser beam toward a second space between the second test pad and the second conductive path, to make the first and second spaces conductive. The detector has a sampler connected to one of the first and second conductive paths, to sample electrical characteristic values of the circuit board.

This is a divisional of application Ser. No. 08/251,953 filed May 31,1994, now U.S. Pat. No. 5,680,056.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit board tester, and moreparticularly, to a tester for testing conduction and insulation betweenoptional pads of a circuit board in a noncontact manner.

2. Description of the Related Art

Circuit boards increasingly have a large number of printed conductivewirings (which may be called as traces or nets, and hereinafter referredto as traces) and a large number of pads connected to the traces. Thisincreases the time required for testing conduction and insulation amongthe pads. Contact testers must prepare contact probes suitable for acircuit board to be tested and control the contact probes on test padsof the circuit board. When the pads are very small, it is difficult toform an array of contact probes suitable for the pads and simultaneouslyand surely bring the probes in contact with the pads. It is necessary,therefore, to provide a noncontact tester to speedily test a circuitboard for conduction and insulation among pads thereof. In particular,it is required to provide a tester for testing a high-density circuitboard such as a multichip module (MCM) board for conduction andinsulation among pads thereof.

Circuit board testing techniques are disclosed in, for example, JapaneseUnexamined Patent Publication Nos. 3-295476, 3-118484, and 4-236367.

The Publication JUPP 3-295476 (first prior art) discloses a contacttester employing a test head in which many metal contact probes areembedded. The probes simultaneously come into contact with many pads ona circuit board. A signal is applied to two arbitrary pads through thecorresponding probes, and a voltage or current between the probes ismeasured to calculate resistance between the two pads. According to thecalculated resistance, it is determined whether or not conductionbetween the pads is allowable if the pads are on the same trace, orwhether or not insulation between the pads is allowable if the pads areon different traces.

This technique is difficult to apply to circuit boards involving veryfine wiring and pads, or a great number of pads. Accordingly, a flyingprobe technique and a two- or four-probe technique that separately movetwo to four metal probes to successively measure resistance betweenevery pair of pads, have been developed.

The Publication JUPP 3-118484 (second prior art) discloses a noncontacttester. This tester emits an electron beam to charge an arbitrary padand a trace connected to the pad to a given voltage, to see whether ornot the voltage appears on another pad or trace. If the voltage appearson another pad on the same trace, it is determined that conduction isgood. If pads on another trace maintain the same voltage level beforeand after the charging, it is determined that insulation is good.

The JUPP 4-236367 (third prior art) discloses a noncontact testeremploying a laser beam and a photoconductive sheet having a transparentconductive film. The tester emits a laser beam so that the conductivefilm is electrically connected to a test pad through a part of thephotoconductive sheet where the laser beam irradiates, to thereby chargethe test pad. Thereafter, the tester emits a laser beam to measurecharges at another pad through the conductive film, thereby determiningthe quality of conduction and insulation between the two pads.

These prior arts have problems, however, as described below.

According to the first prior art, it is nearly impossible to fabricate atest head having an array of probes to deal with several thousands toseveral tens of thousands of pads of a high-density circuit board withthe pads each extending several tens of micrometers square and beingarranged at pitches of about 10 micrometers. It is impossible tocorrectly bring the probes into contact with the pads. Instead of anarray of probes, four discrete probes may be employed to surely makecontact with pads and apply and measure voltages. This technique,however, takes a very long time for testing. When testing 2000 pads,approximately 500 hours will be needed to carry out 2000×2000 insulationtests because the probes need at least 0.5 seconds to measure resistancebetween each pair of the pads.

The second prior art employing electron beams may not have this kind ofproblem but it has another problem. The size of a circuit board isusually 10 to several tens of centimeters square, so that the testermust be three to four meters square to accommodate the circuit board ina vacuum chamber. In the vacuum chamber, the circuit board requires along degassing time, so that it takes about one hour to start the test.

According to the third prior art, the photoconductive sheet and testpads must be completely in contact with each other. This is verydifficult because the circuit board has irregularities of severalmicrometers.

The second and third prior arts charge an optional test pad and observea charged state at another pad due to leakage from the charged pad. Thesecond and third prior arts have a principle problem that it isdifficult to measure correct resistance between the pads because chargeand discharge time constants are affected not only by the resistance butalso by the electrostatic capacitance of traces on which the pads arecontained.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus forcarrying out conduction and insulation tests on arbitrary pads of acircuit board at high speed without employing contact probes, nor avacuum degassing system, nor physical contact with pads. This apparatusis not affected by the electrostatic capacitance of traces of thecircuit board. This apparatus reduces a delay in gate wiring andimproves the operation speed of a semiconductor device.

According to the present invention, there is provided an apparatus fortesting a circuit board having wiring patterns and traces with pads,comprising a unit for holding the circuit board; a unit for detectingthe electrical characteristics of the circuit board, having a conductivepath forming unit spaced apart from the circuit board by a predeterminedgap, to form a first conductive path between a position corresponding toa first test pad on one of the traces and a first power source as wellas a second conductive path between a position corresponding to a secondtest pad on another of the traces and a second power source; a unit forcontrolling a laser plasma switch in a first space between the firsttest pad and the first conductive path and a laser plasma switch in asecond space between the second test pad and the second conductive.path, the laser plasma switches being activated with laser beams to makethe first and second spaces conductive; and an electrical characteristicvalue sampling unit included in the electrical characteristics detectionunit and connected to one of the first and second conductive paths.

The conductive path forming unit may be made from laser transmissionmaterial. The conductive path forming unit may be made fromphotoconductive material. The conductive path forming unit may be madefrom photoconductive material and has a unit for supporting thephotoconductive material. The photoconductive material support unit mayhave a function of controlling light transmission. The photoconductivematerial support unit may be a liquid crystal mask. The photoconductivematerial support unit may have a unit for controlling patterns totransmit light.

The laser plasma switch control unit may have a unit for emitting laserbeams and a unit for deflecting the laser beams. The electricalcharacteristics detection unit may have a unit for exciting thephotoconductive material.

The laser plasma switch control unit may make the first and secondspaces conductive, and a predetermined time thereafter, the electricalcharacteristics detection unit may drive the sampling unit to detect anelectrical characteristic value in the conductive paths. The electricalcharacteristics detected by the electrical characteristics detectionunit may include resistance, voltages, currents, and insulationresistance. The apparatus may further comprise a unit such as an O-ringto seal space between a photoconductive sheet or glass plate and thecircuit board, and the sealed space may be filled with a pressurized ordepressurized air or rare gas such as argon or xenon.

According to the present invention, there is also provided an apparatusfor testing a circuit board having wiring patterns and traces with pads,comprising a photoconductive sheet positioned above the circuit boardwith a predetermined gap therebetween; a light transmission patterncontrol unit arranged above the photoconductive sheet, for controllingthe shape of light emitted from a light source, to form a firstconductive path and a second conductive path on the photoconductivesheet; a first laser plasma switch control unit for controllingconduction between a first test pad of the circuit board and an end ofthe first conductive path; a second laser plasma switch control unit forcontrolling conduction between a second test pad of the circuit boardand an end of the second conductive path; and a first resistancemeasurement unit for measuring resistance between the other end of thefirst conductive path and the other end of the second conductive path.

Further, according to the present invention, there is provided anapparatus for testing a circuit board having metal wiring patterns andtraces with pads, comprising a photoconductive sheet positioned abovethe circuit board with a predetermined gap therebetween; first andsecond transparent comb electrodes formed on the photoconductive sheet,the teeth of the comb electrodes alternating; a first light beam controlunit for emitting and controlling a light beam of predetermined shape toform a first conductive path extending from the first comb electrode toa first test pad of the circuit board; a second light beam control unitfor emitting and controlling a light beam of predetermined shape to forma second conductive path extending from the second comb electrode to asecond test pad of the circuit board; a unit for controlling a firstlaser plasma switch that controls conduction between the first test padand an end of the first conductive path; a unit for controlling a secondlaser plasma switch that controls conduction between the second test padand an end of the second conductive path; and a unit for measuring aresistance value between the other end of the first conductive path andthe other end of the second conductive path through the first and secondcomb electrodes.

According to the present invention, there is provided an apparatus fortesting a circuit board having wiring patterns and traces with pads,comprising a unit for holding the circuit board; a unit for detectingthe electrical characteristics of the circuit board, having a conductivepath forming unit provided with a conductive area, the conductive areabeing spaced apart from the circuit board by a predetermined gap andfacing an area of the circuit board where all test pads are positioned,the conductive area being composed of a plurality of conductive sectionsthat are electrically isolated from one another, one of the conductivesections being connected to a first power source, and the otherconductive sections being connected to a second power source whosepotential is lower than that of the first power source, the conductivesections forming conductive paths between the test pad positions and thepower sources; a unit for controlling laser plasma switches that areformed in at least two spaces between the test pads and the conductivesections, the laser plasma switches being activated when irradiated withlaser beams; and an electrical characteristic value sampling unitcontained in the electrical characteristics detection unit and connectedto one of the conductive paths.

In addition, according to the present invention, there is provided anapparatus for testing a circuit board having wiring patterns and traceswith pads, comprising a unit for holding the circuit board; a unit fordetecting the electrical characteristics of the circuit board, having aunit for facing one face of the circuit board with a predetermined gapbetween them, for forming a first conductive path between a positioncorresponding to a first test pad on a trace of the circuit board and afirst power source, and a unit for facing the other face of the circuitboard with the predetermined gap therebetween, for forming a secondconductive path between a position corresponding to a second test pad ona trace, which is the same or different from the trace containing thefirst test pad, and a second power source whose potential is lower thanthat of the first power source; a unit for controlling a laser plasmaswitch acting in a first space between the first test pad and the firstconductive path as well as a laser plasma switch acting in a secondspace between the second test pad and the second conductive path, thefirst and second laser plasma switches being excited with laser beams,to make the first and second spaces conductive; and an electricalcharacteristic value sampling unit contained in the electricalcharacteristics detection unit and connected to one of the first andsecond conductive paths.

According to the present invention, there is provided a method oftesting a circuit board having wiring patterns and traces with pads inan apparatus, having a unit for holding the circuit board, wherein themethod comprises the steps of positioning the apparatus away from thecircuit board held by the holding unit by a predetermined gap; forming afirst conductive path between a position corresponding to a first testpad on a trace of the circuit board and a first power source; forming asecond conductive path between a position corresponding to a second testpad on another trace of the circuit board and a second power source;emitting a laser beam toward a first space between the first test padand the first conductive path and another laser beam toward a secondspace between the second test pad and the second conductive path, tomake the first and second spaces conductive; and testing the electricalcharacteristics of the circuit board with a sampling unit connected toone of the first and second conductive paths.

According to the present invention, there is also provided a method oftesting a circuit board having wiring patterns and traces with pads,wherein the method comprises the steps of positioning a photoconductivesheet above the circuit board with a predetermined gap therebetween;employing a light transmission pattern control unit arranged above thephotoconductive sheet, to control the shape of light emitted from alight source and form a first conductive path and a second conductivepath on the photoconductive sheet; employing a first laser plasma switchcontrol unit to control conduction between a first test pad of thecircuit board and an end of the first conductive path; employing asecond laser plasma switch control unit to control conduction between asecond test pad of the circuit board and an end of the second conductivepath; and employing a resistance measurement unit to measure aresistance value between the other end of the first conductive path andthe other end of the second conductive path.

Further, according to the present invention, there is provided a methodof testing a circuit board having metal wiring patterns and traces withpads, wherein the method comprises the steps of positioning aphotoconductive sheet above the circuit board with a predetermined gaptherebetween; arranging first and second transparent comb electrodes onthe photoconductive sheet, the teeth of the comb electrodes alternating;employing a first light beam control unit to emit and control a lightbeam having a predetermined shape to form a first conductive pathextending from the first comb electrode to a first test pad of thecircuit board; employing a second light beam control unit to emit andcontrol a light beam having a predetermined shape to form a secondconductive path between the second comb electrode and a second test padof the circuit board; employing a first laser plasma switch control unitto control conduction between the first test pad and an end of the firstconductive path; employing a second laser plasma switch control unit tocontrol conductive between the second test pad and an end of the secondconductive path; and employing a resistance measurement unit to measurea resistance value between the other end of the first conductive pathinvolving the first comb electrode and the other end of the secondconductive path involving the second comb electrode.

According to the present invention, there is provided a method oftesting a circuit board having wiring patterns and traces with pads inan apparatus, having a unit for holding the circuit board, wherein themethod comprises the steps of positioning a conductive area facing thecircuit board with a predetermined gap therebetween, to cover an area ofthe circuit board where all test pads are disposed; dividing theconductive area into a plurality of conductive sections that areelectrically isolated from one another; connecting at least one of theconductive sections to a first power source; connecting another of theconductive sections to a second power source whose potential is lowerthan that of the first power source; forming conductive paths betweentest pad positions and the power sources; employing a laser plasmaswitch control unit to emit laser beams toward spaces between at leasttwo test pads and the conductive sections and make the spacesconductive; and employing an electrical characteristics detection unitto detect the electrical characteristics of the conductive paths.

Further, according to the present invention, there is also provided amethod of testing a circuit board having wiring patterns and traces withpads in an apparatus, having a unit for holding the circuit board,wherein the method comprises the steps of arranging a first conductivesection facing the circuit board held by the holding unit with apredetermined gap therebetween, to secure conduction between a positionof a first test pad on a trace of the circuit board and a first powersource; arranging a second conductive section facing the circuit boardheld by the holding unit with a predetermined gap therebetween, tosecure conduction between a position of a second test pad on anothertrace of the circuit board and a second power source, the secondconductive section being isolated from the first conductive section;employing a laser plasma switch control unit to emit a laser beam towarda first space between the first test pad and the first conductivesection and another laser beam toward a second space between the secondtest pad and the second conductive section, to make the first and secondspaces conductive; and employing an electrical characteristic valuesampling unit connected to at least one of the first and secondconductive sections, to test the electrical characteristics of thecircuit board.

In addition, according to the present invention, there is provided amethod of testing a circuit board having wiring patterns and traces withpads in an apparatus having a unit for holding the circuit board,wherein the method comprises the steps of forming a first conductivepath between a position corresponding to a first test pad on a trace onone face of the circuit board and a first power source; forming a secondconductive path between a position corresponding to a second test pad ona trace, which is the same or different from the trace containing thefirst test pad, on another face of the circuit board and a second powersource whose potential is lower than that of the first power source;employing a laser plasma switch control unit to emit a laser beam to afirst space between the first test pad and the first conductive path andanother laser beam toward a second space between the second test pad andthe second conductive path, to make the first and second spacesconductive; and employing an electrical characteristic value samplingunit connected to one of the first and second conductive paths, tomeasure electrical characteristic values of the circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from thedescription of the preferred embodiments as set forth below withreference to the accompanying drawings, wherein:

FIG. 1 shows a principle of an apparatus for testing a circuit boardaccording to a first aspect of the present invention;

FIG. 2 explains operations of the apparatus of FIG. 1;

FIG. 3 explains operations of the apparatus of FIG. 1;

FIG. 4 explains a principle of measuring resistance between padsaccording to the present invention;

FIG. 5 explains a principle of measuring resistance between padsaccording to the present invention;

FIGS. 6A and 6B show an apparatus for testing a circuit board accordingto an embodiment of the first aspect of the present invention;

FIG. 7 shows waveforms of test sequences carried out by the apparatus ofthe present invention;

FIG. 8 shows a principle of an apparatus for testing a circuit boardaccording to a second aspect of the present invention;

FIG. 9 is an enlarged view showing part of FIG. 8;

FIG. 10 shows transparent conductive film patterns of an apparatus fortesting a circuit board according to an embodiment of the second aspectof the present invention;

FIG. 11 shows the transparent conductive film patterns of FIG. 10 inuse;

FIG. 12 shows a principle of an apparatus for testing a circuit boardaccording to a third aspect of the present invention;

FIG. 13 is a sectional view showing a circuit board tested by theapparatus of FIG. 12;

FIG. 14 explains operations of the apparatus of FIG. 12;

FIG. 15 explains a principle of measuring resistance between pads by theapparatus of FIG. 12;

FIG. 16 explains a principle of measuring resistance between pads by theapparatus of FIG. 12;

FIGS. 17A and 17B show an apparatus for testing a circuit boardaccording to an embodiment of the third aspect of the present invention;

FIG. 18 explains an operation of positioning a light beam according tothe embodiment of FIGS. 17A and 17B;

FIG. 19 shows waveforms of test sequences according to the embodiment;

FIG. 20 explains a case requiring no light beam according to theembodiment;

FIG. 21 explains a measuring technique according to the embodiment;

FIGS. 22A and 22B are flowcharts showing steps of testing a circuitboard according to the third aspect of the present invention;

FIGS. 23A to 23D show essential parts of an apparatus for testing acircuit board according to a fourth aspect of the present invention,wherein FIG. 23A is a plan view showing a path forming unit of thecircuit board tester, FIG. 23B is a plan view showing a parallel shiftof the path forming unit; and FIGS. 23C and 23D are sectional viewsshowing the path forming unit;

FIG. 24 explains a principle of measuring electric characteristicsaccording to the fourth aspect of the present invention;

FIGS. 25A to 25C explain a method of measuring insulation resistancebetween pads according to the fourth aspect of the present invention,wherein FIG. 25B shows pairs of test pads;

FIG. 26 is a block diagram showing the apparatus according to the fourthaspect of the present invention;

FIG. 27 shows waveforms of test sequences according to the fourth aspectof the present invention;

FIG. 28 is a plan view showing a circuit board tested according to thefourth aspect of the present invention;

FIG. 29A shows relationships between the positions of a slit of theapparatus of the fourth aspect of the present invention and test pads;

FIG. 29B shows relationships between pairs of test pads and thepositions of the slit;

FIGS. 30A to 30C are flowcharts showing steps of measuring the electriccharacteristics of a circuit board according to the fourth aspect of thepresent invention;

FIG. 31 shows relationships between the positions of pads and thecoordinates of a slit according to the fourth aspect of the presentinvention;

FIG. 32 explains a relationship between the moving range of a slit andthe pitches of pads according to the fourth aspect of the presentinvention;

FIG. 33A is a plan view showing a path forming unit of an apparatus fortesting a circuit board according to a fifth aspect of the presentinvention;

FIG. 33B is a sectional view showing the path forming unit of FIG. 33A;

FIG. 34A is a plan view showing a positional relationship between thepath forming unit and a circuit board according to the fifth aspect ofthe present invention;

FIG. 34B shows relationships between the positions of the path formingunit of FIG. 33A and power supply and detection pads and test pads;

FIG. 35 is a side view showing another path forming unit according tothe fifth aspect of the present invention;

FIG. 36 is a sectional view showing essential part of an apparatus fortesting a circuit board according to a sixth aspect of the presentinvention; and

FIG. 37 is a general side view showing the apparatus according to thesixth aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be explained in detailwith reference to the-drawings.

FIG. 1 shows a principle of an apparatus for testing a circuit boardaccording to the first aspect of the present invention. The circuitboard 9 has traces such as NA and NB each involving pads such as "TESTPAD IN" and "TEST PAD OUT".

The apparatus has a holder 191, a detector 193, and a laser plasmaswitch controller 196. The holder 191 has a casing 90 and spacers 81 and82, to hold the circuit board 9.

The detector 193 has a path forming unit 192 spaced away from thecircuit board 9 by a predetermined distance. The path forming unit 192forms a first conductive path 71 between a position S1 corresponding toa first pad, e.g., the pad IN on a trace of the circuit board 9 and ahigh-potential first power source V1, as well as a second conductivepath 72 between a position S2 corresponding to a second pad, e.g., thepad OUT on another trace of the circuit board 9 and a low potentiallevel V2 that is at, for example, a ground level lower than the level ofthe first power source V1.

The laser plasma switch controller 196 emits laser beams 10 and 20toward a first space 194 between the first pad IN and the firstconductive path 71 and a second space 195 between the second pad OUT andthe second conductive path 72, to make the first and second spaces 194and 195 conductive.

The detector 193 has an ammeter or voltmeter voltmeter or an ammeterconnected to one of the first and second conductive paths 71 and 72, forobtaining electrical characteristic values.

The apparatus involves first and second lasers 1 and 2, the first andsecond laser beams (pulse laser beams) 10 and 20, first and seconddeflection mirrors (galvanomirrors) 11 and 12, and deflection mirrors11' and 12' cooperating with the first and second deflection mirrors 11and 12. The apparatus also involves a light source 3, a scan lens 4, aliquid crystal mask controller 5, a liquid crystal mask 6, aphotoconductive sheet 7, uniform light 30 provided by the light source 3to excite the photoconductive sheet 7, an optical axis 41 of the scanlens 4, and the spacers 81 and 82.

The circuit board 9 includes a ground layer 91, a power source layer 92,a conduction defect D1, and an insulation defect D2. The laser beams 10and 20 excite plasma switches P1 and P2 in the spaces 194 and 195.

The trace NA involves the pad IN and the trace NB involves the pad OUT.The circuit board 9 has the conduction defect D1 to be detected by aconduction test and the insulation defect D2 to be detected by aninsulation test.

The photoconductive sheet 7 or a glass plate coated with aphotoconductive film is arranged above the circuit board 9 with a gap ofseveral tens of micrometers between them. The liquid crystal mask 6 forselectively passing light beams is fixed to the photoconductive sheet 7.The scan lens 4 and deflection mirrors 11 and 12 separately converge,deflect, and position the laser beams 10 and 20 in X and Y directions(the Y direction is not shown). The light source 3 is used to excite thephotoconductive sheet 7. The lasers 1 and 2 emit pulse laser beamstoward the deflection mirrors 11 and 12, respectively.

FIGS. 2 and 3 explain operations of the apparatus of FIG. 1 whenmeasuring resistance between the pads IN and OUT and determining whetheror not insulation between the pads is good.

The liquid crystal mask controller 5 forms light transmission patterns61 and 62 in the liquid crystal mask 6, as shown in FIG. 2. The light 30from the light source 3 transmits the patterns 61 and 62 and irradiatesthe photoconductive sheet 7. As a result, the conductive paths 71 and 72corresponding to the patterns 61 and 62 are formed on thephotoconductive sheet 7. The conductive path 71 extends from an inputpad (a first electrode pad) Io to a position just above the pad IN. Theconductive path 72 extends from a position just above the pad OUT to anoutput pad (a second electrode pad) Oo.

The pulse laser beams 10 and 20 are controlled to irradiate the padpositions indicated with black dots in FIG. 2. The laser beams 10 and 20excite the plasma switches P1 and P2 in the spaces 194 and 195 as shownin FIG. 3, to momentarily make the input path 71 conductive with the padIN, and the output path 72 conductive with the pad OUT. This forms anelectrical path from the input pad Io to the pad IN and an electricalpath from the pad OUT to the output pad Oo.

Accordingly, a current flows from a voltage source connected to theinput pad Io, passes through the pads IN and OUT, and reaches the outputpad Oo. This current is dependent on resistance (in this case,insulation resistance) between the pads IN and OUT. Accordingly, aresistance value Rmes between the pads Io and Oo is calculable bymeasuring the output current just after the application of the pulselaser beams. Then, an insulation resistance value Rin-out between thepads IN and OUT is calculable by subtracting resistance values Rin andRout of the paths 71 and 72, and twice an ON resistance value Rlps ofthe laser plasma switch, from the resistance value Rmes.

FIGS. 4 and 5 explain a principle of measuring resistance between padsof a circuit board by the apparatus of the present invention, in. whichFIG. 4 shows changes in resistance values, voltages, and currents atvarious parts after forming input and output paths and emitting pulselaser beams, and FIG. 5 shows an equivalent circuit to measure aninsulation resistance value between a pair of pads (IN and OUT). Thelaser beams 10 and 20 are simultaneously emitted toward the pads IN andOUT, to excite, i.e., to lower resistance values of the laser plasmaswitches P1 and P2.

When a laser is emitted toward two conductors, plasma is producedbetween the conductors, which are electrically connected to each otheraccordingly. This is a known ionization phenomenon of gas such as air,argon, and xenon due to a laser beam excitation. This phenomenon isdescribed in, for example, "Laser Engineering," pp. 207 onward, December1972, supervised by Yoshihiro Asami, Publishing Dept. of Tokyo DenkiUniversity. According to this document, the power of a laser beam ofdischarging a gas and generating plasma is dependent on the wavelengthof the laser beam. With a wavelength in the range of 500 to 700nanometers, the laser beam needs maximum power to produce plasma.Namely, this wavelength hardly produces enough plasma to turn on a laserplasma switch. With a wavelength over one micrometer or below 300nanometers, the laser beam needs little power to produce plasma. In thiscase, the laser plasma switch is turned on with laser beam power ofabout 1/5 to 1/10 of the former case. Accordingly, the present inventionemploys a laser beam of one to two micrometers in wavelength, to providelarge output power.

Don L. Millard et al. disclose a switch employing this phenomenon in"Noncontact Testing of Circuits Via a Laser-Induced Plasma ElectricalPathway," IEEE Design & Test of Computers, March 1992, p 55. Thistechnique places an electrode having a center opening opposite to anelectrode to be measured. A pulse laser beam is emitted through theopening of the electrode, to produce laser plasma to make the openingelectrode conductive with the objective electrode. The voltage of theopening electrode, i.e., a waveform applied to the objective electrodeis observed on an oscilloscope for several tens of microseconds to 100microseconds.

The apparatus according to the first aspect of the present inventionforms laser plasma switches on a photoconductive sheet and forms aclosed circuit with optional conductive patterns in a noncontact manner.The closed circuit is employed to speedily and accurately measureresistance.

Referring to FIGS. 4 and 5, a problem of the noncontact measuringtechnique is that resistance to be measured is affected by electrostaticcapacitance values Cin and Cout of traces. According to the first aspectof the present invention, the influence of the electrostatic capacitanceis instantaneous on rise times "trin" and "trout." In a predeterminedtime after emitting pulse laser beams, voltage and current values becomepurely dependent on insulation resistance. If an ON period of the laserplasma switch is about one millisecond, the electrostatic capacitancewill never influence the measurement of resistance if a current issampled about 0.5 milliseconds after emitting pulse laser beams.

The resistance values Rin and Rout are easily calculable according to anexperimentally obtained resistance value per unit length of thephotoconductive sheet 7 and the length of the path 71 or 72 calculatedfrom data used for forming the patterns 61 and 62 on the liquid crystalmask 6. The ON resistance Rlps of the laser plasma switch isexperimentally obtained. The accuracy of measurement of the resistancebetween the pads IN and OUT is influenced by fluctuations in theresistance values Rin, Rout, and Rlps. These resistance values can besuppressed to several kilohms, to provide corresponding accuracy inmeasuring the resistance. Namely, the present invention is capable ofcarrying out a test with an accuracy of several kilohms, and thisaccuracy is quite sufficient to test whether or not insulation is good.

FIGS. 6A and 6B shows an apparatus for testing a circuit board accordingto an embodiment of the first aspect of the present invention. Theapparatus involves a first laser 1, a second laser 2, first and secondlaser beams 10 and 20, galvanomirrors 11, 11', 12, and 12' fordeflecting the laser beams 10 and 20, a light source 3, a scan lens 4, aliquid crystal mask 6, a photoconductive sheet 7, and a circuit board 9to be tested. The light source 3 emits uniform light 30 to excite thephotoconductive sheet 7. The apparatus also includes insulation spacers81 and 82, a casing 90, and an insulation sheet 93, to form a holder forholding the circuit board 9.

The apparatus also involves an XYZ stage 100, a first laser positioncontroller 101, a second laser position controller 102, a timingcontroller 103, a constant voltage source 104, an ammeter 105, an A/Dconverter 106, a liquid crystal mask controller 107, an XYZ stagecontroller 108, and a controller 109.

The galvanomirrors 11 and 12 deflect the laser beams 10 and 20,respectively. The galvanomirrors 11 and 12 are equally distanced from anoptical axis 41 of the scan lens 4 on a plane orthogonal to the opticalaxis 41 and adjacent to a focus of the scan lens 4. The light source 3is arranged between the galvanomirrors 11 and 12.

The controller 107 controls the liquid crystal mask 6 to form conductivepaths 71 and 72 on the photoconductive sheet 7. The constant voltagesource 104 is connected to an input pad Io of the sheet 7. The currentdetector (ammeter) 105 is connected to an output pad Oo of the sheet 7.The first pulse laser beam 10 is positioned on a first test pad A, andthe second pulse laser beam 20 on a second test pad B. The pulse laserbeams 10 and 20 are simultaneously emitted. Just after the emission ofthese beams, a current is measured. These processes are repeated todetermine whether or not insulation of the circuit board 9 is good.

Operations of the embodiment of FIGS. 6A and 6B will be explained inconnection with an insulation test.

The circuit board 9 is placed on the insulation sheet 93 in the casing90. The insulation spacers 81 and 82 are placed on the circuit board 9.The casing 90 with the circuit board 9 is mounted on the XYZ stage 100.The coordinate systems of the liquid crystal mask 6 and deflectors areadjusted in relation to each other in advance. These coordinate systemsare adjusted in relation to the coordinate system of the circuit board9. The constant voltage source 104 is always connected to the input padIo, and the ammeter 105 is connected to the output pad Oo. The lightsource 3 is turned on.

The liquid crystal mask controller 107 controls the transmissivity ofeach pixel of the liquid crystal mask 6, to form a light transmissionpattern 61 extending from the input pad Io to the first test pad A and alight transmission pattern 62 extending from the second test pad B tothe output pad Oo. These patterns are formed according to design datafor the circuit board 9 stored in the controller 109. It is preferableto form the patterns to be as short as possible. The uniform light 30transmits only the light transmission patterns 61 and 62 of the liquidcrystal mask 6, to form the input path (first conductive path) 71 andoutput path (second conductive path) 72 on the photoconductive sheet 7.The photoconductive sheet 7 may be made from gallium arsenide (GaAs),amorphous silicon, or photoconductive material employed byelectrophotography. The liquid crystal mask 6 may employ an opticalsystem to condense or expand transmission light onto the photoconductivesheet 7.

According to the design data of the circuit board 9 stored in thecontroller 109, the first laser position controller 101 positions thefirst laser beam 10 on the test pad A, and the second laser positioncontroller 102 positions the second laser beam 20 on the test pad B. Thetiming controller 103 provides a pulse signal for controlling the firstand second laser beams 10 and 20, to make a space 194 between the inputpath 71 and the pad A and a space 195 between the pad B and the outputpath 72 conductive for a predetermined interval. This interval is, forexample, about several hundreds of microseconds to one millisecond.

In the embodiment of FIGS. 6A and 6B, the laser beams 10 and 20 areemitted from the lasers 1 and 2, respectively. These beams may beprovided by dividing a common laser beam using a beam splitter. Each ofthe lasers 1 and 2 emits a pulse laser beam of about one micrometer inwavelength. Each of the lasers 1 and 2 may be a mode lock laser, and ashutter for intermittently blocking a laser beam is provided by the modelock laser. Alternatively, each of the lasers may be a Q-switch laser.

About 0.5 milliseconds after the emission of the laser beams 10 and 20,the timing controller 103 provides a sampling trigger signal. Inresponse to this signal, the A/D converter 106 converts an outputcurrent value into digital data, which is transmitted to the controller109. According to the data, the controller 109 calculates a resistancevalue between the test pads A and B. To compensate for the resistance ofthe input and output paths 71 and 72, the lengths of the paths 71 and 72are calculated. To measure resistance between the input and output padsIo and Oo of the photoconductive sheet 7, an AC signal of sine wave orpulse wave is applied to the input pad Io, and the maximum amplitude ofan output current waveform provided by the output pad Oo relative toground is detected. The resistance between the input and output pads Ioand Oo is calculated according to a ratio between the input voltage, orthe amplitude of the input voltage waveform and the detected outputcurrent value.

This completes the insulation test between the test pads A and B. Inpractice, the pad A is successively tested with respect to pads C, D,and so on on other traces. The liquid crystal mask 6 is controlled tochange only the output path pattern 62. At the same time, only theposition of the second laser beam 20 is changed to, for example, the padC, and the processes mentioned above are repeated.

FIG. 7 shows waveforms showing test sequences according to the firstaspect of the present invention. These sequences are only models. Inpractice, the output path 72 is changed to deal with the next test pad Cafter sampling an output current related to the present test pad. Inthis case, a value detected by the ammeter suddenly drops to zero whenthe output path 72 is changed. Each cycle of the sequences for measuringa resistance value between a pair of test pads requires about one to twomilliseconds. After all pairs involving the pad A are tested, the firstlaser beam 10 is shifted to the next pad, and the sequences mentionedabove are repeated.

This embodiment arranges the lasers, scan mirrors (galvanomirrors), andscan lens in this order. It is possible to arrange the lasers,convergent lenses, and scan mirrors in this order.

As explained above, the apparatus according to the first aspect of thepresent invention employs the electrically controllable liquid crystalmask to form input and output paths. Laser beams are emitted,simultaneously or one after another, toward two test pads on a circuitboard, to electrically connect the input path to one of the pads and theoutput path to the other pad. Then, conduction or insulation between thepads is tested in a noncontact manner.

Namely, the apparatus according to the first aspect of the presentinvention employs the technique of freely forming input and output pathson a photoconductive sheet to connect input and output pads to test padsof a circuit board, and the technique of activating laser plasmaswitches with laser beams, to make spaces between the input and outputpaths and the test pads conductive. With the combination of thesetechniques, the first aspect of the present invention instantaneouslymeasures resistance between the input and output pads and calculatesresistance between the test pads.

Accordingly, the first aspect of the present invention shortens aninsulation test time, eliminates contact probes that are difficult tofabricate and unreliable in contact performance, and needs no vacuumdegassing system. This aspect of the present invention involves nophysical contact with test pads and measures resistance between the testpads with no influence of the electrostatic capacitance of the tracesthat contain the test pads. It is easy to narrow a laser beam to ten-oddmicrometers and make the spatial resolution of a plasma switch equal tothe length of a gap between a test pad and a conductive path.Accordingly, this aspect is capable of carrying out the insulation teston pads each of several tens of micrometers that are difficult to testby the multiple-probe tester of the prior art. The two- or four-probetester according to the prior art requires mechanical movement for everytest, to consume at least several hundreds of milliseconds perinsulation test. On the other hand, the first aspect of the presentinvention completes the test within several milliseconds, to speed upthe testing time at least by one order of magnitude compared with theprior art.

The apparatus according to the first aspect of the present inventionbasically requires no contact probes, nor vacuum degassing system, norphysical contact between test pads and a test head including transparentelectrodes. The first aspect forms a closed circuit with a voltagesource, power supplying input pad, photoconductive input path, firsttest pad, second test pad, photoconductive output path, output pad, andgrounding electrode. According to a voltage applied to the circuit and acurrent flowing through the circuit, the first aspect measures aresistance value between the test pads with no influence of theelectrostatic capacitance of the traces that contain the test pads.

A testing time is influenced by a laser plasma switching time, laserdeflection time, and the S/N ratio of detected signals. The laser plasmaswitching time, i.e., the ON time of a laser plasma switch is in therange of 10 microseconds to one millisecond, and the settling time of adeflected laser beam is several hundred microseconds. Accordingly, ameasurement of the resistance of a pair of test pads will be finishedwithin one to two milliseconds, which is about 1/200 of 0.5 secondsrequired by the four-probe tester of the prior art.

The conductive path forming unit 192 preferably employs laser beamtransmitting material, more preferably, photoconductive material.

The conductive path forming unit 192 preferably employs thephotoconductive sheet 7 and a support 6' for supporting the sheet 7. Thesupport 6' is preferably the liquid crystal mask 6 having a lighttransmission control function.

In this case, it is preferable to provide the support 6' with the lighttransmission pattern controller 5.

The detector 193 cooperates with the laser plasma switch controller 196,which involves the lasers 1 and 2 and deflectors 11 and 12.

The detector 193 includes the light source 3 for exciting thephotoconductive sheet 7.

The detector 193 drives the laser plasma switch controller 196 to makethe first and second spaces 194 and 195 conductive, and after apredetermined time, drives the sampler 197 to detect an electricalcharacteristic value in the conductive paths 71 and 72. The electricalcharacteristics detected by the detector 193 will be at least one ofresistance, voltage, current, and insulation resistance.

An apparatus for testing a circuit board according to the second aspectof the present invention will now be explained.

The first aspect of the present invention explained above has somedisadvantages. The ON resistance of the photoconductive sheet irradiatedwith light is relatively high and reaches from several hundred kilohmsto several megohms when the length of a path formed on thephotoconductive sheet extends for several tens of millimeters to severalhundred millimeters. This ON resistance is an obstacle in the conductivepath. When the photoconductive sheet is made from erbium (Er) dopedGaAs, the ON resistance of a conductive path of one micrometer in lengthis about 100 ohms. According to a simple proportional calculation, itwill reach several hundred kilohms if the length is several millimeters,and several megohms if the length is several tens of millimeters. Whenthe ON resistance is several megohms, it widely fluctuates in the rangeof several hundred kilohms. This ON resistance is connected in serieswith resistance to be tested, to fluctuate the accuracy of the test byabout several hundreds of kilohms.

Accordingly, the second aspect of the present invention forms aphotoconductive sheet 7 with a plurality of transparent conductive filmpatterns. This shortens the actual length of a conductive path to beformed and reduces the ON resistance thereof.

FIG. 8 corresponds to FIG. 2 and explains the principle of the secondaspect of the present invention. FIG. 9 is an enlarged view showing partof FIG. 8.

In FIG. 8, the photoconductive sheet 7 according to the second aspect ofthe present invention is made from ITO (In203:Sn). Discrete transparentconductive film patterns 70 are formed at intervals of about onemicrometer over the sheet 7. The size of each of the patterns 70 is, forexample, 20 micrometers square. This arrangement reduces the actuallength of a conductive path by the ratio of each pattern 70 to a spacebetween the patterns 70.

In FIGS. 8 and 9, light transmission patterns 61 and 62 are formed on aliquid crystal mask 6, and conductive patterns 71 and 72 correspondingto the patterns 61 and 62 are formed on the photoconductive sheet 7. Thefilm patterns 70 contained at least partly in the conductive paths 71and 72 form the paths. More precisely, film patterns 70a, 70b, 70c, and70d of FIG. 9 around the conductive path 71 have substantially the samepotential as that of the conductive path 71. However, they cause noproblem. The conductive path 71 extends from an input pad Io to aposition just above a first test pad IN, and the conductive path 72 froma position just above a second test pad OUT to an output pad Oo.

In each of the conductive paths 71 and 72, the transparent conductivefilm patterns 70 have low resistance, and the photoconductive sheet 7having relatively large resistance exists only between the film patterns70. As a result, the ON resistance of the conductive paths 71 and 72 isreduced to 100 kilohms to several kilohms, i.e., one several tenths toseveral hundredths of that of the first aspect.

Similar to the first aspect, resistance values Rin and Rout of the paths71 and 72 are easily obtained by experimentally finding the resistanceof the conductive paths per unit length and by calculating the length ofthe paths 71 and 72 according to control data of the mask 6. When thepaths 71 and 72 are made of Er-doped GaAs film patterns, the values Rinand Rout are reduced to several hundred kilohms or smaller due to theeffect of the discrete film patterns 70 formed over the photoconductivesheet 7. The ON resistance Rlps of a laser plasma switch isexperimentally obtained.

The accuracy of measurement of a resistance value between test pads isinfluenced by fluctuations in the resistance values Rin, Rout, and Rlps.Each of these resistance values is about several tens of kilohms, and afluctuation in the resistance value is estimated as several tens ofkilohms, that is about 10% of the resistance value. Accordingly, aconduction test may be carried out at an accuracy of about several tensof kilohms, which is sufficient for an insulation test.

FIG. 10 shows transparent conductive film patterns 70' according toanother embodiment of the second aspect of the present invention, andFIG. 11 shows an application of the film patterns of FIG. 10.

In FIG. 10, the discrete film patterns 70' are formed at predeterminedpitches over a photoconductive sheet 7. The patterns 70' are made fromITO (In203:Sn). Each of the patterns 70' has a comb shape to mesh withfour adjacent patterns 70'. The length L1 of a tooth 700 is, forexample, one fourth the length Lo of the pattern 70'. The length Lo is,for example, ten micrometers to several tens of micrometers. Thisarrangement increases the effective width of a conductive path betweenthe adjacent patterns 70' by several times that of the rectangularpatterns 70 of FIG. 8. This results in decreasing ON resistance to afraction of that of the patterns 70.

In FIG. 11, a light transmission mask (liquid crystal mask) 6 forms alight pattern 61, which forms a conductive path 71.

The spatial accuracy of the conductive path 71 is determined by the sizeof each pattern 70'. Accordingly, this embodiment causes an error ofabout 1/2 of the size of the pattern 70' in the accuracy of the path 71.If the size of the pattern 70' is 20 micrometers, the accuracy of thepath 71 is within 10 micrometers. This provides sufficient spatialresolution because even a high-density circuit board involves a wiringwidth of about 50 micrometers and a pad of about 50 micrometers.

In this way, the apparatus according to the second aspect of the presentinvention employs a photoconductive sheet covered with transparentconductive film patterns, to reduce the actual length of a conductivepath to be formed on the photoconductive sheet and lower the ONresistance of the path. More precisely, the second aspect of the presentinvention reduces the ON resistance by one to two orders of magnitudecompared with that of the first aspect of the present invention. Thesecond aspect, therefore, improves the accuracy of the insulationresistance test.

An apparatus for testing a circuit board according to the third aspectof the present invention will now be explained.

FIG. 12 shows a principle of the apparatus according to the third aspectof the present invention, FIG. 13 is a sectional view showing a circuitboard to be tested, and FIG. 14 explains operations of the apparatus ofFIG. 12.

The apparatus involves first and second laser beams (pulse laser beams)10 and 20, first and second light beams 301 and 302 for exciting aphotoconductive sheet 7, first and second transparent comb electrodes 51and 52, first and second photoconductive patterns 701 and 702, and acircuit board 9 to be tested. The circuit board 9 includes a groundlayer 91, a power source layer 92, a conduction defect D1, and aninsulation defect D2. Marks P1 and P2 indicate plasma zones (laserplasma switches) excited by the laser beams 10 and 20, respectively.

FIG. 13 shows a first trace NA including a test pad i (IN) and a secondtrace NB including a test pad j (OUT). The conduction defect D1 will befound by a conduction test, and the insulation defect D2 will be foundby an insulation test. A space between a glass plate 70 and the circuitboard 9 may be sealed airtight with O-rings and filled with air or arare gas such as argon or xenon under increased or decreased pressure.

The glass plate, i.e., test head 70 is coated with the photoconductivesheet 7 on which the first transparent comb electrode 51 serving as apower supply electrode and the second transparent comb electrode 52serving as a detection electrode are formed. The test head 70 ispositioned above the circuit board 9 and is spaced away from it byseveral tens of micrometers. The first light beam 301 irradiates thetest head 70, to form the first photoconductive pattern 701, and thesecond light beam 302 irradiates the test head 70, to form the secondphotoconductive pattern 702. The first and second laser beams 10 and 20also irradiate the test head 70, to activate the laser plasma switchesP1 and P2.

Principle operations of measuring resistance between the first andsecond test pads i and j, to determine whether or not the test pads iand j are well insulated from each other will be explained next.

The first light beam 301 is emitted to cover the first test pad i andpart of the first transparent comb electrode 51 nearest to the firsttest pad i, to form the first photoconductive pattern 701. At the sametime, the second light beam 302 is emitted to cover the second test padj and a part of the second transparent comb electrode 52 nearest to thesecond test pad j, to form the second photoconductive pattern 702. Thisforms a conductive path extending from a power supply pad Io to aposition just above the first test pad i, and a conductive pathextending from a position just above the second test pad j to adetection pad Oo.

The pulse laser beams 10 and 20 are simultaneously emitted to the padpositions indicated with large black dots in FIG. 12. The pulse laserbeams 10 and 20 excite the laser plasma switches P1 and P2 between thetest head 70 and the circuit board 9. As a result, the comb electrodes51 and 52 become momentarily conductive to the test pads i and j,respectively, to form an electrical path extending from the power supplypad Io, passing through the test pads i and j, and reaching thedetection pad Oo.

Each of the lasers 1 and 2 may comprise a mode lock laser for carryingout a continuous pulse operation and a shutter for intermittentlyblocking a laser beam provided by the laser. Alternatively, each of thelasers 1 and 2 may be a Q-switch laser.

A current from a voltage source 204 connected to the power supply pad Ioflows through the test pads i and j and detection pad Oo and reachesground. This current is dependent on an insulation resistance value Rijbetween the test pads i and j. A resistance value Rmes between the powersupply pad Io and the detection pad Oo is calculable by measuring theoutput current when the pulse laser beams are emitted or atpredetermined timing after the emission of the pulse laser beams. Theinsulation resistance value Rij is calculable by subtracting resistancevalues Rin and Rout of the electrodes and conductive paths and doublethe ON resistance Rlps of the laser plasma switch, from the resistancevalue Rmes. Namely, the insulation resistance value Rij is calculableaccording to the following formula:

    Rij=Rmes-Rin-Rout-2×Rlps

FIGS. 15 and 16 explain a principle of measuring resistance between testpads according to the third aspect of the present invention. Thesefigures correspond to FIGS. 4 and 5. Namely, the test pads IN and OUT,rise times "trin" and "trout," insulation resistance Rin-out, and theelectrostatic capacitances Cin and Cout of the pads IN and OUT of FIGS.4 and 5 correspond to the test pads i and j, rise times "tri" and "trj,"insulation resistance Rij, and the electrostatic capacitances Ci and Cjof the pads i and J. Accordingly, the explanations of these elementswill not be repeated.

The third aspect of the present invention combines the laser plasmaswitch technique with the technique of forming rectangularphotoconductive patterns on a photoconductive sheet, to form a closedcircuit in a noncontact manner, as shown in FIG. 16. With this closedcircuit, the third aspect accurately measures resistance between testpads at high speed. The conventional noncontact technique has a problemthat the measurement of resistance is influenced by the electrostaticcapacitances Ci and Cj of traces. According to the present invention,the influence of the electrostatic capacitances occurs only during therise times tri and trj, as shown in FIG. 15. In a predetermined timeafter emitting pulse laser beams, a voltage value and a current valuebecome purely dependent on insulation resistance. The rise time tri isabout 0.5 microseconds when Rin+Rlps is about 10 kilohms and theelectrostatic capacitance Ci is about 50 picofarads at the most. Therise time trj is about 0.5 milliseconds when the resistance Rij is 10megohms in the case of good insulation. Accordingly, a measurement willnot be affected by the electrostatic capacitances, if the measurement isdone about 0.5 milliseconds after emitting pulse laser beams and if theON time of the laser plasma switch is about one millisecond.

The resistance values Rin and Rout are simply obtainable according tothe experimentally obtained resistance values of the comb electrodes andconductive paths per unit length and the lengths of the comb electrodesand conductive paths calculated according to control data forpositioning the beam patterns and the coordinates of the test pads. Thetypical values of the Rin and Rout employing Er-doped GaAs as thephotoconductive sheet are each several kilohms to several tens ofkilohms for a length of several hundred micrometers. The Rlps is alsoexperimentally obtained and is typically several kilohms. Accuracy ofmeasuring resistance between test pads is mainly influenced byfluctuations in Rin, Rout, and Rlps. Each of these resistance values isabout several tens of kiloohms and a fluctuation therein is aboutseveral kilohms, that is about 10% of the resistance value. Accordingly,a conduction test may be carried out at an accuracy of several kilohms,which is sufficient for an insulation test.

FIGS. 17A and 17B show an apparatus for testing a circuit boardaccording to another embodiment of the third aspect of the presentinvention. The apparatus involves a first laser 1, a second laser 2, alaser beam 10 emitted from the laser 1, a laser beam 20 emitted from thelaser 2, a two-dimensional deflector 11 for deflecting the first laserbeam 10, a two-dimensional deflector 21 for deflecting the second laserbeam 20, a first light source 31 for exciting a photoconductive sheet 7,and a second light source 32 for exciting the photoconductive sheet 7.The apparatus also involves filter lens systems 311 and 321, rectangleslits 312 and 322, two-dimensional deflectors 313 and 323, a scan lens4, the photoconductive sheet 7, a glass plate 70, and a circuit board 9to be tested. The apparatus also involves insulation spacers 81 and 82,a casing 90, an insulation sheet 93, and a test head support 900. Theapparatus also involves an XYZ stage 200, a first laser positioncontroller 201, a second laser position controller 202, a timingcontroller 203, a constant voltage source 204, an ammeter 205, an A/Dconverter 206, a laser pulse generation controller 207, an XYZ stagecontroller 208, and a controller 209. The apparatus also involves afirst light beam position controller 210 for controlling a first lightbeam 301 to excite the photoconductive sheet 7, a second light beamposition controller 211 for controlling a second light beam 302 toexcite the photoconductive sheet 7, a first test pad data file 212, anda second test pad data file 213.

The elements 311, 312, 313, 321, 322, and 323 form, project, and deflectthe two rectangular beams 301 and 302 to form conductive paths. Theelements 11 and 21 deflect the two laser beams 10 and 20 to activatelaser plasma switches. The constant voltage source 204 is connected to apower supply pad Io, and the ammeter 205 is connected to a detection padOo. The first pulse laser beam 10 is positioned at a first test pad i,and the second pulse laser beam 20 is positioned at a second test pad j.The laser beams 10 and 20 are simultaneously emitted. A current ismeasured just after the emission of the laser beams. These processes arerepeated to carry out an insulation test.

The insulation test carried out by the apparatus according to the thirdaspect of the present invention will be explained next. The elementsrelated to the laser beam 10 and 10 and light beams 301 and 302 arearranged above the test head 70 and are fixed to a frame (not shown).The circuit board 9 is placed on the insulation sheet 93 in the casing90. The insulation spacers 81 and 82 are placed on the circuit board 9so that it is spaced apart from the test head 70 by several tens ofmicrometers. The casing 90 with the circuit board 9 is mounted on theXYZ stage 200. The coordinate systems of the test head and deflectorsare adjusted in relation to each other in advance. These coordinatesystems are adjusted in relation to the coordinate system of the circuitboard 9. The voltage source 204 is always connected to the power supplypad Io, and the ammeter 205 is connected to the detection pad Oo.

The rectangular light beam 301 forms a rectangular conductive pattern701 extending from part of a power supplying transparent conductive filmcomb pattern 51 to the test pad i. The rectangular light beam 302 formsa rectangular conductive pattern 702 extending from the test pad j topart of a detection transparent conductive film comb pattern 52. Each ofthe light sources 31 and 32 is, for example, a tungsten lamp thatgenerates white light. The white light is filtered by the filter lenssystems 311 and 321 into light of about 800 nanometers in wavelength.The filtered light is shaped by the rectangular slits 312 and 322. Thepositioning of the conductive patterns 701 and 702 is controlledaccording to design data of the circuit board 9 stored in the controller209 and data related to the combinations and coordinates of test padsstored in the files 212 and 213, so that the patterns 701 and 702 coverparts of the comb patterns 51 and 52 nearest to the test pads i and J.

FIG. 18 explains the positioning of the light beams 301 and 302 andshows an essential part of the test head 70. The width (Yk, u-Yk, l) ofeach of the comb patterns 51 and 52 is several tens of micrometers,similar to the size of each of the test pads i and j. A pitch P of theteeth of the comb patterns 51 and 52 is several times the width andpreferably several hundreds of micrometers so as not to excessivelyincrease the resistance of the conductive patterns 701 and 702. In FIG.18, the width of each of the comb patterns 51 and 52 is 40 micrometersand the pitch P thereof is 240 micrometers.

As is apparent in FIG. 18, a conductive path to a position just above agiven pad is not unique. When the first and second test pads i and j aredistanced away from each other, the rectangular conductive pattern 701is positioned such that the comb pattern 51 to be connected to the testpad i (xi, yi) comes to approximately the center of the pattern 701.When the two test pads i and j are close to each other, it is necessaryto position the two conductive patterns 701 and 702 so that they do notoverlap each other. If the length of each of the conductive patterns 701and 702 is equal to the pitch P of the comb patterns 51 and 52, theconductive pattern 701 or 702 is able to cover all pads between adjacentteeth of the comb patterns 51 and 52.

In this way, the rectangular light beams 301 and 302 to form theconductive patterns 701 and 702 are positioned and irradiate the testhead 70, to form power supply and detection paths extending from thecomb patterns 51 and 52 to the test pads i and j. The photoconductivefilms 7, 51, and 52 may be made from GaAs, Cr- or Er-doped GaAs,amorphous silicon, or phthalocyanine-based photoconductive material usedby electrophotography.

Referring again to FIGS. 17A and 17B, the first and second laserposition controllers 201 and. 202 refer to design data of the circuitboard 9 stored in the controller 209, to position the first laser beam10 at the test pad i and the second laser beam 20 at the test pad j. Thetiming controller 203 provides a pulse signal to generate the pulselaser beams 10 and 20, so that a space between a power supply pathformed of the comb pattern 51 and conductive pattern 701 and the testpad i and a space between the test pad j and a detection path formed ofthe comb pattern 52 and conductive pattern 702 become conductive for apredetermined time. This time is usually in the range of severalhundreds of microseconds to one millisecond.

The timing controller 203 provides a sampling trigger signal about 0.5milliseconds after the emission of the laser beams 10 and 20. Accordingto the trigger signal, the A/D converter 206 converts an output currentvalue into digital data, which is transferred to the controller 209.According to the data, the controller 209 calculates a resistance valueRij between the test pads i and j. At this time, the resistance of thepower supply and detection paths must be compensated by calculating thelengths of the comb patterns 51 and 52 and conductive patterns 701 and702 involved.

The insulation test between the test pads i and j is thus completed. Inpractice, the first test pad i is successively tested with respect toother test pads j+1, j+2, and so on, to improve test efficiency. Forthis purpose, the position of the rectangular light beam 302 for forminga detection path and the position of the laser beam 20 for activatingthe laser plasma switch for the second test pad are changed, and theabove steps are repeated.

It is possible to apply an AC signal to the input pad Io, detect themaximum amplitude of an output current waveform provided by the outputpad Oo to ground, and calculate resistance between the pads Io and Ooaccording to a ratio between the amplitude of the input voltage waveformand the maximum amplitude of the output current waveform.

FIG. 19 shows model waveforms of test sequences according to the thirdaspect of the present invention.

In practice, the pad j is changed to the next pad j+1 just after thesampling of an output current for the pad j. In this case, a detectionvalue of the ammeter 205 suddenly drops to zero. Resistance between apair of pads is measured within one to two milliseconds. After allinsulation tests for the test pad i are completed, the first laser beam10 is shifted to the next test pad i+1, and the same sequences arerepeated. This embodiment arranges the lasers, two-dimensionaldeflectors such as galvanomirrors, and scan lens (convergent lens) inthis order. They may also be arranged in order of the lasers, convergentlens, and two-dimensional deflectors. According to the embodiment, thelight beam sources 31 and 32 emit white light, and the filters 311 and321 provide light of required wavelength. It is possible to employlasers.

In practice, the third aspect of the present invention must considerspecial positional relationships between test pads and the comb patterns51 and 52, as shown in FIGS. 20 and 21.

FIG. 20 shows a case that requires no light beam and FIG. 21 shows acase that requires a special measure.

In FIG. 20, test pads i and j are positioned just below the combpatterns 51 and 52, respectively. Accordingly, it is not necessary toemit the rectangular light beams 301 and 302. Only emitting the firstand second laser beams 10 and 20 toward the test pads i and j will do.In this case, a voltage must be applied to the test pad i and theammeter must be connected to the test pad J. These connections areirreversible.

The case of FIG. 21 is impossible to test as it is. Test pads i and jare both positioned just under the comb pattern 51, so that power supplyand detection paths are not separately formed. This kind of specialcombination of test pads is stored as separate test data in a file. Atfirst, normal combinations of test pads are tested, and then, thecircuit board 9 is shifted in a direction Y by half the pitch P of theteeth of the comb patterns 51 and 52. As a result, the test pads i and jcome out from under the comb pattern 51, and the test pads i and j arenormally tested.

FIGS. 22A and 22B are flowcharts showing steps of testing the circuitboard 9 according to the third aspect of the present invention.

Step ST1 determines whether or not coordinates yi and yj of test pads iand j are just under one of the comb patterns 51 and 52. Namely, thestep ST1 checks to see whether or not Yku<=yi<=Yk1 and Yku<=yj<=Yk1(k=1, 2, . . . N).

If the step ST1 provides YES, step ST3 registers the pads i and j in thesecond test pad data file 213, and if NO, step ST2 registers the pads iand j in the first test pad data file 212. Step ST4 determines whetheror not all test pads have been checked. If NO, the flow returns to thestep ST1, and if YES, step ST5 is carried out. For the step ST4, allcombinations of the test pads must be registered in advance.

The step ST5 successively reads test data such as the coordinates oftest pads pair by pair out of the first file 212. Step ST6 positions therectangular light beams 301 and 302 and the pulse laser beams 10 and 20.Step ST7 measures resistance between the test pads i and j. Step ST8stores a result of the measurement. Step ST9 determines whether or notall test pads in the first file 212 have been tested. If NO, the flowreturns to the step ST5, and if YES, the flow goes to step ST10.

To deal with the unmeasurable case of FIG. 21, the step ST10 shifts thecircuit board 9 in the direction Y by half the pitch P of the teeth ofthe comb patterns 51 and 52. Step ST11 adds P/2 to the y-coordinate dataof the test pads stored in the second file 213. Step ST12 reads thecoordinates of the pads pair by pair out of the second file 213. StepST13 positions the rectangular light beams 301 and 302 and the pulselaser beams 10 and 20. Step ST14 measures resistance between the testpads i and j. Step ST15 stores a result of the measurement. Step ST16determines whether or not all pads stored in the second file 213 havebeen tested. If NO, the flow returns to the step ST12, and if YES, thetest flow ends.

In this way, the third aspect of the present invention arranges the twotransparent conductive film comb patterns 51 and 52 with their teethalternating each other on the photoconductive sheet 7. A power supplypad is formed on the periphery of one of the comb patterns, and adetection pad on the periphery of the other. Power supply and detectionpaths extending from the power supply and detection pads to test pads ofa circuit board are optionally electrically formed and changed with useof the comb patterns 51 and 52 and the photoconductive effect of therectangular beams 301 and 302. The pulse laser beams 10 and 20 areemitted to turn on laser plasma switches in spaces between the powersupply and detection paths and the test pads, to make the spacesconductive. Then, resistance between the power supply and detection padsis quickly measured to calculate resistance between the test pads.

Next, circuit board detector according to the fourth aspect of thepresent invention will be explained in detail with reference to thedrawings.

According to the first to third aspects of the present invention, thepath forming unit 192 employs a photoconductive film and a patternedmask or a liquid crystal shutter for forming proper patterns. The lasercontroller 196 emits light, which transmits the patterns formed on themask or shutter, to form first and second conductive paths on thephotoconductive film.

Namely, the first to third aspects must prepare conductive paths foreach pair of test pads, and for this purpose, must have the mask orliquid crystal shutter and a driver for driving the shutter. Thisresults in extending a testing time and increasing costs.

The apparatus according to the fourth aspect of the present inventionsolves this problem by employing a simple path forming unit toefficiently and speedily test a circuit board having wiring patterns andtraces with pads.

The apparatus has a holder for holding the circuit board, a detector fordetecting the electric characteristics of the circuit board, and a laserplasma switch controller. The detector has the path forming unitpositioned away from the circuit board by a predetermined distance. Thepath forming unit has a conductive area facing an area of the circuitboard where all test pads are positioned. The conductive area is formedof conductive sections electrically isolated from one another. At leastone of the conductive sections is connected to a first power source, andanother to a second power source whose potential is lower than that ofthe first power source. The path forming unit forms conductive pathsbetween predetermined test pad positions and the power sources. Thelaser plasma switch controller emits laser beams toward spaces betweenthe two test pads and the conductive sections, to make the spacesconductive. The detector has an electrical characteristic value samplerconnected to the conductive paths.

The arrangement of this apparatus is basically the same as that of anyone of the first to third aspects except for the path forming unit 192.

Namely, the path forming unit 192 of the fourth aspect is made of theflat conductive area. Unlike the first to third aspects of the presentinvention that individually form conductive paths having requiredshapes, the fourth aspect employs the conductive area as the conductivepaths.

The conductive area of the fourth aspect of the present invention isflat to entirely cover a circuit board to be tested. The conductive areais made of at least two conductive sections that are electricallyisolated from each other. Separation of the conductive sections is notparticularly limited. For example, they may be separated with a slit.When the conductive area is made from photoconductive material, theconductive sections may be separated with a light blocking material.

FIGS. 23A to 23D explain the path forming unit 192 of the apparatus fortesting a circuit board according to the fourth aspect of the presentinvention. The path forming unit 192 employs a conductive area dividedinto two conductive sections with a slit.

Except the path forming unit 192, the apparatus of the fourth aspect isthe same as that of the first aspect of FIG. 1. The circuit board 9tested by the apparatus has traces with pads. The path forming unit 192is included in a detector 193 for detecting the electric characteristicsof the circuit board 9. The path forming unit 192 is spaced apart, by apredetermined distance, from the circuit board 9 held by a holder 191.The path forming unit 192 involves first and second conductive sections261 and 262 that are isolated from each other with a slit 263. The firstconductive section 261 conducts a position S1 corresponding to a firsttest pad IN on a trace A of the circuit board 9 to a first power sourceV1. The second conductive section 262 conducts a position S2corresponding to a second test pad OUT on a trace B of the circuit board9 to a second power source V2.

Other arrangements of this apparatus are the same as those of FIG. 1.When the first and second conductive sections 261 and 262 are formedfrom conductive material, the light source 3 of FIG. 1 is not required.In this case, only the laser plasma switch controller involving thelasers 1 and 2, and deflectors are needed, and the conductive sections261 and 262 must transmit laser beams.

When the conductive sections 261 and 262 are made from photoconductivematerial, the light source unit 3 to excite the material and the laserplasma switch controller are needed, similarly to the arrangement ofFIG. 1.

The fourth aspect of the present invention will be explained next inmore detail with reference to FIGS. 23A to 23D.

A glass plate 264 and a transparent conductive film 267 form a testhead. The narrow center slit 263 divides the conductive film 267 intothe first and second conductive sections 261 and 262. The periphery ofthe first conductive section 261 has a power supply pad 265, and theperiphery of the second conductive section 262 has a detection pad 266.The test head is placed such that the conductive film 267 faces thecircuit board 9 with a predetermined gap between them. The firstconductive section 261 is connected to the first voltage source V1, andthe second conductive section 262 is connected to an ammeter 268, whichis connected to the second voltage source V2. The conductive sections261 and 262 are mechanically moved orthogonal to the slit 263 so thatone of two optional test pads IN and OUT (i and j) is positioned underthe first conductive section 261 and the other is positioned under thesecond conductive section 262, to form a power supply path and adetection path to the pads IN and OUT. Laser plasma switches P1 and P2to be excited with pulse laser beams are employed to conduct the powersupply and detection paths to the test pads. With these techniques, thefourth aspect of the present invention instantaneously measuresresistance between the power supply and detection pads 265 and 266 andcalculates resistance between the two test pads IN and OUT.

FIG. 23D is a sectional view showing the circuit board 9. The circuitboard 9 includes the trace A with the first test pad i and the trace Bwith the second test pad j. This board 9 involves a conduction defect D1to be tested by a conduction test and an insulation defect D2 to betested by an insulation test.

FIGS. 23A and 23B are front views showing the test head and circuitboard, and FIGS. 23C and 23D are sectional views showing the same. Thecircuit board 9 has traces A to D and test pads E1 to E8. The size ofthe glass plate 264 is double the size of the circuit board 9. Thetransparent conductive sections 261 and 262 separated by the center slit263 cover the lower face of the glass plate 264. The conductive sections261 and 262 are above the circuit board 9 and are spaced away from thecircuit board 9 by a gap of several micrometers to several tens ofmicrometers. The first conductive section 261 is connected to thevoltage source V1 through the power supply pad 265, and the secondconductive section 262 is connected to the second power source V2, i.e.,is grounded through the detection pad 266 and ammeter 268.

The pulse laser beams 10 and 20 are simultaneously emitted, the beam 10being emitted toward the test pad i, i.e., El in the trace A, and thebeam 20 being emitted toward the test pad j, i.e., E4 in the trace B, toexcite the laser plasma switch P1 in the space between the firstconductive section 261 and the pad i and the laser plasma switch P2 inthe space between the second conductive section 262 and the pad j. As aresult, the conductive section 261 momentarily becomes conductive withthe pad i, and the conductive section 262 becomes conductive with thepad j, to form an electrical path passing through the power supply pad265, test pads i and j, and detection pad 266. In this state, a currentfrom the voltage source V1 flows through the power supply pad 265, testpads i and j, and detection pad 266 and reaches the grounding powersource V2. This current is dependent on resistance (insulationresistance) between the test pads i and j. The output current ismeasured when or after the pulse laser beams are emitted, to findresistance Rmes between the power supply and detection pads 265 and 266.Then, insulation resistance Rij between the test pads i and j isobtained by subtracting resistance values Rin and Rout of the conductivesections 261 and 262, and double the ON resistance Rlps of the laserplasma switch, from the resistance value Rmes. Namely, the insulationresistance Rij is obtained as follows:

    Rij=Rmes-Rin-Rout-2×Rlps

FIG. 24 is an enlarged section showing the operations of the fourthaspect of the present invention.

Measuring operations of the fourth aspect of the present invention arethe same as those of FIG. 4, and an equivalent circuit of the fourthaspect is the same as that of FIG. 5. Accordingly, their explanationswill not be repeated.

A method of testing a circuit board according to the fourth aspect ofthe present invention will be explained next.

This method is a combination of the laser plasma switch technique andthe technique of moving the test head with the conductive sections 261and 262 relative to the circuit board 9, to select an optional pair oftest pads positioned under the two conductive sections 261 and 262,respectively, thereby forming a closed circuit in a noncontact manner asshown in FIG. 24. Then, the resistance between the test pads isaccurately and speedily measured.

The first to third aspects of the present invention employ apredetermined mask or a liquid crystal shutter, to test insulationresistance between test pads i and j. When light is emitted, the mask orshutter forms conductive paths of predetermined patterns on aphotoconductive film.

Namely, the first to third aspects of the present invention must form aconductive path between the first power source V1 and the first test padi and another conductive path between the second power source V2 and thesecond test pad j. This may complicate the apparatus and increase thecost thereof. In addition, the process of forming such conductive pathselongates a testing time.

In addition, for every pair of test pads, conductive paths havingdifferent lengths, widths, and shapes must be prepared and theresistance of each path must be measured. Even if the conductive pathsare formed in the same shape, they will involve fluctuations inresistance. Accordingly, it is necessary to check the fluctuations. Thisinvolves additional operations which deteriorate operability.

Accordingly, the fourth aspect of the present invention does not formindividual conductive paths for every pair of test pads. Instead, thefourth aspect employs the conductive area 270 formed of the conductivesections 261 and 262 each having a predetermined resistance value. Theconductive sections 261 and 262 serve as conductive paths.

The conductive sections 261 and 262 are electrically separated from eachother with the slit or light blocking material 263. When measuringinsulation resistance between two test pads i and j, one of the testpads is positioned under the conductive section 261, and the other testpad under the conductive section 262.

After the insulation resistance between the test pads is measured, theconductive area 270 is shifted along the circuit board 9, to coveranother pair of test pads. Once the slit 263 is properly positioned forthe test pads, insulation resistance between the test pads is measured.

The conduction area 270 is configured to be moved relative to thecircuit board 9 along the-surface of the circuit board 9 orthogonal tothe slit 263.

Some pairs of test pads will not be covered with the unidirectionalmovement of the slit 263. Accordingly, it is preferable to arrange atleast one of the conductive area 270 and circuit board 9 to be turnableby 90 degrees while keeping them in parallel.

The testing method according to the fourth aspect of the presentinvention will be explained with reference to FIGS. 25A to 25C.

The slit 263 of the conductive area, i.e., test head 270 is moved totest every pair of pads on the circuit board 9. In FIG. 25A, the slit263 is longitudinal between the conductive sections 261 and 262. Thecircuit board 9 has test pads E1 to E6 any one of which will berepresented as a test pad i or j. The power supply pad, detection pad,and laser beams are not shown in the figures. A minimum pitch of thepads on the circuit board 9 is P. The slit 263 is laterally shifted andpositioned at one of positions k=1 to k=9 that are plotted at intervalsof P/2. When the slit 263 is at the position k=1, the slit 263 dividesthe pads into a left group of E1 and E2 and a right group of E3 to E6.One of the pads in the left group is paired with any one of the pads inthe right group and tested. Namely, as shown in FIG. 25B, the pad i(=1), i.e., E1 is tested with respect to the pad j (=3 to 6), and thepad i (=2), i.e., E2 is tested with respect to the pad j (=3 to 6). Whenthe slit 263 is shifted to the position k=3, the pads i=3 and i=4 aretested with respect to the pads j=5 and j=6. In this way, the slit 263is shifted up to the position k=9. The pads such as E1 and E2, and E3and E4 linearly arranged along the slit 263 must be tested with respectto each other.

Accordingly, the slit 263 is turned by 90 degrees as shown in FIG. 25C,to test the pads that are not testable with the longitudinal slit 263 ofFIG. 25A. Namely, pairs of the pads shown in the bottom table of FIG.25B are tested with lateral slit positions. In this case, the slit 263is mechanically moved linearly two times. This may take a little time.This time, however, is short compared with that needed fortwo-dimensional movements of the conventional flying prober.

For example, with a circuit board of 100 millimeters square, a minimumpitch P of test pads of 0.04 millimeters, each shift of the test head270 of 0.02 millimeters, and a shift time of the test head 270 of 0.2seconds, the total time for moving the test head 270 10000 times isabout 33 minutes.

FIG. 26 shows an apparatus for testing a circuit board according to anembodiment of the fourth aspect of the present invention.

A path forming unit 192 serving as a test head is made of a glass plate273, which is coated with transparent conductive films 271 and 272 suchas ITO films. The films 271 and 272 are separated from each other with acenter slit 263. The test head is mounted on a test head stage 274 thatis movable in left and right directions in the figure. The test headstage 274 is mounted on a rotary stage 275 that it turnable by 90degrees. Namely, the slit 263 is moved by mechanically moving the testhead.

The width of the slit 263 must be as small as possible, preferably 1/10to 1/20 of a minimum pitch P of the test pads, i.e., about 5 micrometersor less. The slit 263 is moved step by step at fixed intervals of aboutP/2. Left and right ends of the test head are provided with power supplyand detection pads, respectively, through which first and secondconductive sections 261 and 262 made of the conductive films 271 and 272are connected to a voltage source and an ammeter 268, respectively.

An insulation sheet 276 is placed on a board stage 191 on which thecircuit board 9 is placed. The board stage 191 is positioned under thetest head 192. Gaps P1 and P2 are formed between the circuit board 9 andthe test head 192. The gaps P1 and P2 must be as small as possible tosecure spatial resolution and reduce the ON resistance of a plasmaswitch. The gaps P1 and P2, however, must be as large as possible toreduce fluctuations in the ON resistance of the plasma switch due toirregularities of the surface of the circuit board 9. When the circuitboard 9 has irregularities of several micrometers and when theirregularities must be less than 10% of the true size of the gaps P1 andP2, the gaps must be more than several tens of micrometers. On the otherhand, the gaps P1 and P2 must not be more than several tens ofmicrometers, in order to measure a pad whose size is several tens ofmicrometers. Accordingly, the size of the gaps is determined to beseveral tens of micrometers, typically 30 micrometers.

In FIG. 26, two lasers 1 and 2 simultaneously emit two pulse beams 10and 20. Laser deflection systems 11 and 12 arranged above the test head192 are capable of positioning the laser beams 10 and 20 at anypositions on the test head 192. The timing of emitting the laser beamsis controlled by a timing controller 278 according to an instructionfrom a controller 277. The timing controller 278 provides an A/Dconverter 279 with a trigger signal that is behind the laser emissiontrigger signal by a predetermined delay time. In response to the triggersignal, an instantaneous value of a detected current signal is fetchedas digital data by the controller 277.

The controller 277 has two test pad data files 280 and 281 for storingcombinations of test pads and the coordinate data thereof. Thecontroller 277 reads the data out of the files 280 and 281, to controlthe laser emission and deflection systems 282 to 285 and emit pulselaser beams toward the test pads i and j. The files 280 and 281 storenot only data related to the test pads but also results of preprocessesto be explained later.

A data file 286 stores design data of wiring of the circuit board 9. Thefile 286 may serve as the files 280 and 281.

This embodiment moves and turns the test head, i.e., the path formingunit 192 relative to the circuit board 9 while maintaining parallelismbetween them. For this purpose, the test head 192 is set on the stage274 that is horizontally movable in parallel with the circuit board 9.The stage 274 is set on the rotary stage 275 that rotatably holds thestage 274, to at least turn the test head 192 relative to the circuitboard 9 by 90 degrees.

The horizontally moving stage 274 is driven by a motor 290, which iscontrolled by an output signal of a motor controller 288.

The rotary stage 275 is driven by a motor 291, which is controlled by anoutput signal of a motor controller 287.

The circuit board 9 may be moved by moving the holder 191 through aproper moving unit controlled by a board stage controller 289.

The apparatus has a frame 293.

An insulation test for measuring resistance between test pads i and jwill be explained next.

The holder 191 and lasers 1 and 2 are fixed to the frame 293. The casing294 accommodates the insulation sheet 276 on which the circuit board 9is placed. The casing 294 with the circuit board 9 is set on the holder191. The coordinate systems of the laser deflection systems 1 and 2 andboard 9 are adjusted in relation to each other (the adjusting means areomitted). The test head stage 274 moves the slit 263 to a firstposition. The coordinate values of the test pads i and j are read out ofthe files 280 and 281. According to the data, the first and second laserposition controllers 283 and 284 position the first laser beam 10 to thepad i and the second laser beam 20 to the pad J. The timing controller278 provides the first and second laser beam generation controllers 282and 285 with a pulse signal for controlling the generation of the firstand second pulse laser beams 10 and 20. As a result, a power supply pathbetween the power supply pad 265 and the first conductive section 261becomes conductive with the pad i, and the pad j becomes conductive witha detection path between the second conductive section 262 and thedetection pad 266, for a certain period. This period is usually in therange of several hundred microseconds to one millisecond.

About 0.5 milliseconds after the laser emission timing, the timingcontroller provides a sampling trigger signal. In response to thissignal, the A/D converter 279 converts an output current value intodigital data, which is transferred to the controller 277. According tothe data, the controller 277 calculates resistance between the pads iand j. At this time, the lengths of the paths in the conductive area 270are calculated to correct the resistance of the power supply anddetection paths.

These processes complete the insulation test on the pads i and j. FIG.27 shows the sequences of the insulation test. A cycle of the sequencesfor measuring resistance between a pair of pads takes about one to twomilliseconds. After the pad i is tested with respect to all other testpads, the first laser beam is shifted to the next pad i+1, to repeat thetest sequences.

Although the laser optical systems of this embodiment have no lenses,lenses such as a scan lens may be inserted as and when required.

Pads on an actual circuit board are usually complicated and irregular inarrangement, as shown in FIG. 28. To test such pads, it is necessary topreprocess slit positions and combinations of the test pads. Thepreprocess includes (1) extracting pairs of pads that are theoreticallyimpossible to test with a longitudinal slit and (2) extracting pairs oftest pads for each slit position.

The preprocess carried out on eight pads shown in FIG. 28 will beexplained with reference to FIGS. 30A to 30C. In FIG. 28, dotted squaresindicate minimum pitches P of the pads arranged on the circuit board,and continuous squares indicate the size S of each pad. The ratio S/P issmaller than 1 and is usually about 0.6. Accordingly, the dotted squareswill never overlap one another.

The first process (1) is considered hereinafter. When the absolute valueof a difference of x-coordinates of every pair of pads is greater thanthe minimum pitch P, all pad pairs are testable with a longitudinalslit. If the absolute value of a difference between x-coordinates of apair of pads is smaller than the minimum pitch P, the absolute value ofa difference between y-coordinates is greater than the minimum pitch P,as is apparent in FIG. 28. In FIG. 30A, pairs of pads that are testablewith the longitudinal slit are stored in the data file 280, and pairs oftest pads that are testable with a lateral slit are stored in the datafile 281. FIGS. 29A and 29B show a result of grouping of pad pairs to betested with the longitudinal and lateral slits.

The testing process will now be explained step by step with reference toFIGS. 30A to 30C.

Step ST21 obtains x-coordinates xi and xj of a pair of test pads i andj. Step ST22 determines whether or not the absolute value of adifference xi-xj is smaller than the minimum pitch P of pads. If NO,step ST23 stores the test pads i and j in the first file 280, and ifYES, step ST24 determines whether or not the absolute difference xi-xjis equal to the minimum pitch P. If YES, step ST25 obtains y-coordinatesyi and yj of the test pads i and j and determines whether or not theabsolute value of a difference yi-yj is smaller than the minimum pitchP. If YES, the step ST23 is carried out, and if NO, step ST26 stores thetest pads i and j in the second file 281. The stored data are used whenthe slit is turned to a lateral position to test the pads.

If NO in the step ST24, the step ST26 is carried out.

Step ST27 determines whether or not the coordinates of all pairs of testpads have been checked. If NO, the step ST22 is repeated, and if YES,step ST28 detects the electrical characteristics such as insulationresistance and wiring breakage of the circuit board.

Namely, the steps up to the step ST27 determine whether or not thelongitudinal slit is sufficient to measure the electric characteristicsof all test pads arranged on the circuit board 9 and whether or not itis necessary to turn the slit to a lateral position to measure theelectric characteristics of the pads.

For the step ST27, pad pairs to be tested are predetermined.

Steps following step ST28 calculate slit coordinates. The step ST28reads data related to a pair of test pads i and j out of the first file280. Step ST29 selects the smaller one of the x-coordinates xi and xj ofthe test pads i and j and represents it as xs.

Step ST30 finds "n" that satisfies the following:

    P/4(2n-1)<xs<P/4(2n-1)

n=1, 2, 3, . . .

Then, a slit coordinate kx=n+1 is stored in the data file.

Step ST31 determines whether or not slit coordinates kx have been setfor every pair of test pads. If NO, the step ST29 is repeated, and ifYES, step ST32 carries out the same processes for the lateral slit.

Namely, the step ST32 reads data related to a pair of test pads i and jout of the second file 281. Step ST33 selects, as ys, a smaller one ofthe y-coordinates yi and yj of the pads i and j.

Step ST34 finds "n" that satisfies the following:

    P/4(2n-1)<ys<P/4(2n-1)

n=p1, 2, 3, . . .

Then, a slit coordinate ky=n+1 is stored in the data file.

Step ST35 determines whether or not slit coordinates ky have been setfor every pair of test pads. If NO, the step ST33 is repeated, and ifYES, step ST36 starts to measure, for example, insulation resistance.

The step ST36 reads data related to a pair of test pads out of the firstfile 280. Step ST37 determines whether or not slit coordinates have beencompletely read. If NO, step ST38 sets a slit coordinate kx, and stepST39 shifts the slit to the coordinate and sets the slit.

Step ST40 positions the lasers 1 and 2, and step ST41 measuresresistance between the test pads.

Step ST42 stores the result of the measurement in a proper storage unit.Step ST43 determines whether or not all pairs of test pads have beentested. If NO, the step ST40 is repeated, and if YES, the step ST37 isrepeated.

If YES in the step ST37, step ST44 turns the slit by 90 degrees. StepST45 reads data related to a pair of test pads i and j out of the secondfile 281. Step ST46 determines whether or not all slit coordinates havebeen read. If YES, the flow ends.

If NO in the step ST46, step ST47 sets a slit coordinate ky, and stepST48 shifts the slit to the coordinate and sets the slit.

Step ST49 positions the lasers 1 and 2, and step ST50 measuresresistance between the test pads.

Step ST51 stores the result of the measurement in the storage unit. StepST52 determines whether or not all test pad pairs related to the slitcoordinate ky have been tested. If NO, the step ST49 is repeated, and ifYES, the flow ends. Thereafter, electrical characteristics such asinsulation resistance are detected, and the state of wire breakage isconfirmed.

FIGS. 29A and 29B show combinations of test pads to be tested with thelongitudinal and lateral slits.

Returning to FIG. 28, the process (2) will be explained. Design rules ofpads on a circuit board specify minimum pitches but freely allow theabsolute positions of the pads or specify the absolute positions in verysmall order, for example, one micrometer. Strictly, the slit must bepositioned at the left end of each pad when carrying out a test. Thisgreatly increases the number of movements of the slit, to deterioratethe merit of high-speed testing with laser beams. In practice, many padsare testable with one slit position.

According to this embodiment, each movement of the slit is half theminimum pad pitch P. The reason for this will be explained withreference to FIGS. 31 and 32. FIG. 31 shows optimum slit positions forpads continuously arranged in the direction x. A dotted square indicatesthe minimum pad pitch P. Vertical lines with white and black dotsindicate slit positions. When the slit coordinates change at intervalsof P/2, part of a pad corresponding to P/4 at the maximum protrudes overthe slit into the other conductive section as shown in FIG. 32. This isonly about 8% of the pad with a space/pitch ratio of 0.6. This is whythis embodiment shifts the slit at intervals of P/2.

A process of finding an optimum slit coordinate with respect to a givenpad coordinate x corresponds to the steps ST28 to ST35 of FIGS. 30A and30B. Slit coordinates at intervals of P/2 are first obtained for pairsof pads to be tested with the longitudinal slit and are stored in thedata file. Then, slit coordinates are obtained for pairs of pads to betested with the lateral slit and are stored in the data file.

In practice, pairs of pads are related to corresponding slit coordinatesand stored as shown in FIGS. 29A and 29B. Actual measurement and testsequences are shown in FIGS. 30A to 30C. Namely, a pair of pads i and jare read out of the first file 280. The slit is shifted according to aslit coordinate corresponding to the pads i and j. The laser beams arepositioned at the pads i and j, and a current is measured. According tothe current, a resistance value between the pads i and j is calculatedto determine whether or not conduction or insulation between the pads iand j is good. The result is then recorded. These processes are repeateduntil all pairs of pads for the same slit position are tested. Then, theslit is shifted to the next slit coordinate. After all pads testable onthe longitudinal slit are tested, the slit is turned to the lateralposition, to repeat the tests.

According to the embodiment explained above, the test head is linearlymoved only twice so that the tests are completed with a very short stagemoving time compared with the conventional flying prober thatmechanically moves the probes two-dimensionally.

When testing a circuit board of 100 millimeters square with a padminimum pitch P of 40 micrometers and a unit movement time of the stageof 0.2 seconds, it takes about 30 minutes to move the stage 10000 times.If the pads are irregularly arranged on the circuit board, the slit maybe shifted in variable steps instead of fixed steps. This will furtherreduce the number of stage movements.

FIG. 33A shows the fifth aspect of the present invention. The fourthaspect of the present invention involves a relatively large number ofslit movements which take several tens of minutes to move a slit stageto test all pads. This time is relatively large. The fifth aspectfurther reduces the number of slit movements.

The fifth aspect of the present invention divides a conductive area of atest head 192 into three sections 353, 354, and 355 with two slits 351and 352. The sections are provided with power supply and detection pads356, 357, and 358, respectively. A relay circuit 361 and a relaycontroller 360 are provided to optionally change connections among thesepads and a voltage source V1 and a current detector 268. The minimumrequired conductive area of the test head is G×H that is provided by thefollowing equation:

    G=NA/(N-1)

    H=A

where A is the size of a circuit board. In the example of FIG. 33A, thenumber of conductive sections N is 3 so that G is 1.5 A.

FIG. 33B is a sectional view showing the arrangement of FIG. 33A.

FIGS. 34A and 34B show operation sequences according to the fifth aspectof the present invention. Pads are distributed over the circuit board 9,as shown in FIG. 34A. For the sake of simplicity of explanation, thetest head is moved at intervals of pitch P of the pads. At a slitcoordinate k=1, the pad 1 is connected to the voltage source PS, and thepads 2 and 3 are connected to the current detector CD. In this case, thepad i=1 is tested with respect to the pads j=2 to 8. When the pad 2 isconnected to the voltage source PS and the pads 1 and 3 are connected tothe current detector CD, the pads i=2 to 5 are tested with respect tothe pad j=6 to 8. Namely, 12 pairs of pads are tested with one slitcoordinate as shown in FIG. 34B. Accordingly, the movement of the slitis equal to the length g of one of the conductive sections at themaximum, which is 1/2 that of the fourth aspect. Namely, the number ofshifts of the slit of the fifth aspect is 1/2 of that of the fourthaspect.

When the transparent conductive film is divided into more sections, thenumber of shifts is further reduced, and the size of the test head maybe reduced. If N=11 for example, the number of shifts becomes 1/10 ofthe former example. Namely, the number of shifts becomes 1000 times andthe moving time becomes 3 minutes.

FIG. 35 shows the sixth aspect of the present invention. This aspectemploys another means to divide a conductive area into sections. A testhead is provided made of a glass plate 363 coated with a photoconductivefilm 362. A glass plate 365 having a thin linear light blocking pattern364, is positioned on the glass plate 363. Uniform coherent lightirradiates the glass plates 363 and 365. The wavelength of the light isabout 0.5 to one micrometer which is appropriate to inducephotoconductivity. The light blocking pattern 364 forms a shadow on thephotoconductive film 362, and the shadow acts as a slit in aphotoconductive area 270. The slit is movable by horizontally moving theglass plate 365.

Similarly to the first to third aspects of the present invention, thefourth and fifth aspects easily condense a laser beam to ten and severalmicrometers, to make the spatial resolution of a plasma switchsubstantially equal to the length of a gap between a test pad and aconductive path. Accordingly, these aspects are capable of carrying outan insulation test on pads each of several tens of micrometers thatcannot be tested by a conventional multiple-probe tester.

The conventional two- or four-probe tester must two-dimensionally move astage, so that it takes several hundreds of milliseconds for testing apair of pads. On the other hand, the present invention takes severalmilliseconds for testing a pair of pads with the stage being linearlymoved. When testing pairs of 1000 pads arranged at minimum pitches of 40micrometers on a circuit board of 100 millimeters square, theconventional tester takes about 40 hours, while the present inventiontakes one hour (the former example) to 30 minutes (the latter example).Namely, the present invention reduces the testing time to at least 1/40of the prior art.

These figures are obtained with a testing speed of 0.3 seconds per pairof pads according to the prior art and a stage moving speed of 0.2seconds per step and a measuring speed of 3 milliseconds per pair ofpads according to the present invention.

According to the fourth and fifth aspects of the present invention, thelaser plasma switch controller makes the spaces conductive, and apredetermined time after that, the sampler detects the electricalcharacteristic values of conductive paths including the conductivesections.

According to the fourth and fifth aspects of the present invention, theslit is moved in one direction to detect the electric characteristics ofevery pair of test pads located on each side of the slit. If there areany pairs of test pads that have not been tested with the one-directionmovement of the slit, their addresses are stored in a proper storageunit. The slit is turned by 90 degrees and the conductive area is movedorthogonal to the slit, to detect the electrical characteristics of thepairs of test pads stored in the storage unit.

Namely, the apparatus according to any one of the fourth and fifthaspects of the present invention tests a circuit board having metalwiring patterns and traces with pads for insulation among the traces andconduction among the pads on each trace.

The apparatus has a transparent test head having at least two conductivesections that are electrically separated from each other with a slit.Each of the conductive sections has at least a metal pad, i.e., a powersupply pad or a detection pad. The test head is spaced apart from thecircuit board by a predetermined gap, so that the two conductivesections face the circuit board.

The apparatus has two laser beam emitters for emitting laser beamshaving a wavelength to transmit through the conductive sections of thetest head to make conductive paths for supplying power and detectingelectrical characteristics, and units for separately converging anddeflecting the laser beams to discrete positions on the circuit board.

The apparatus also has a unit for measuring resistance between the powersupply pad and the detection pad.

The laser beams are positioned at two test pads under the two conductivesections of the test head. The laser beams excite plasma to cause a gasconduction phenomena (laser plasma switches) to electrically connect thepower supply pad of the test head to one of the test pads and thedetection pad of the test head to the other pad.

Resistance between the power supply pad and the detection pad ismeasured, and accordingly, it is determined whether or not conduction orinsulation between the two test pads is good.

The unit for measuring resistance between the power supply pad and thedetection pad includes a unit for applying a constant voltage to thepower supply pad and a detector for detecting an output current flowingfrom the detection pad to ground. Alternatively, the unit may include aunit for applying an AC signal (a sine wave or a pulse wave) to thepower supply pad and a detector for detecting the amplitude of an outputcurrent waveform flowing from the detection pad to ground. Theresistance between the power supply pad and the detection pad iscalculated according to a ratio between the input voltage or theamplitude of the input voltage waveform and the output current or theamplitude of the output current waveform.

The conductive sections may be made of a photoconductive film. The filmis formed on the surface of a transparent support such as a glass platethat faces a circuit board to be tested. A thin linear light blockingstripe is formed on the transparent plate opposite to the circuit board.The glass plate is provided with a unit for horizontally moving theglass plate orthogonally to the blocking stripe. Above the test head, anemitter is arranged for emitting coherent light for uniformlyirradiating the photoconductive film. The coherent light has awavelength of 0.5 to 1 micrometer. The light blocking stripe forms atleast two isolated sections on the photoconductive film.

The test head is moved relative to the circuit board. In this case, somepairs of test pads may come only under the same conductive section. Thiswill happen when such a pair of test pads is arranged on a straight linethat runs in parallel with the slit or stripe. These kinds of pairs oftest pads are extracted before starting measurements. The numbers andcoordinates of such pairs are registered in a second test pad data file.Other normal test pads are registered in a first test pad data file.Each pair of test pads stored in the first file is read out, and the twolaser beams are emitted toward the test pads, to apply a voltage to thetest pads, detect a current, and measure resistance between the pads.The slit is shifted to test every pair of the pads stored in the firstfile.

The circuit board or the test head (slit) is then turned by 90 degrees.At the same time, the coordinates of the test pads stored in the secondfile, or the laser beam deflection coordinate system is alto turned by90 degrees. The same test is carried out on every pair of the test padsstored in the second file.

When the test head involves at least three conductive sections, it ispossible to provide a unit for optionally changing connections of thepower supply and detection pads, which are connected to the conductivesections, to the resistance measuring voltage source and currentdetector.

The length g of the conductive section of the test head orthogonal tothe slit, the width H thereof along the slit, and the total length G ofthe test head are obtained as follows: g>=A/(N-1)

H>=A

G>=N×g=N×A/(N-1)

where A is the size of the square circuit board and N is the number ofthe conductive sections (N=2, 3, . . . ).

The at least three conductive sections of the test head are electricallyinsulated from one another. One of the three conductive sections, i.e.,the first conductive section is connected to the voltage source or tothe current detector, and the others are connected to the currentdetector or to the voltage source. A conduction test or an insulationtest is carried out on pairs of test pads with one pad in each pairbeing under the first section and the other pad under one of the othersections. Thereafter, the second conductive section is connected to thevoltage source or to the current detector, and the others are connectedto the current detector or to the voltage source. The conduction test orinsulation test is carried out on pairs of test pads with one pad ineach pair being under the second conductive section and the other padunder one of the other conductive sections. In this way, the test withapplying a voltage and measuring a current is repeated through differentcombinations of the conductive sections.

The test head is horizontally shifted relative to the circuit board by apredetermined distance orthogonal to the slit. This shift causes newpairs of pads to come under the conductive sections. The conduction orinsulation test is carried out on these pairs of pads.

The test processes are repeated until the test head is shifted relativeto the circuit board by the length g of the conductive section. In thisway, the conduction or insulation test is carried out on all pairs oftest pads on the whole face of the circuit board, to thereby completethe test of the electric characteristics of the circuit board.

Some pairs of test pads will always come under the same conductivesection even if the test head is shifted relative to and in parallelwith the circuit board. This will happen when a pair of test pads isarranged on a straight line that runs in parallel with the slit. Thesekinds of pairs of pads are extracted before starting the test, and thenumbers (i, j) and coordinates of the pads are stored in the secondfile. The numbers (i, j) and coordinates of normal pairs of test padsare stored in the first file. The conduction or insulation test is firstcarried out on the pairs of test pads stored in the first file. Namely,the two laser beams are emitted to the pads pair by pair, to apply avoltage to the pads, a current is detected, and resistance between thepads is measured. The test is repeated until the slit is shifted for thelength g of any one of the conductive sections.

Thereafter, the circuit board or the test head with the slit is turnedby 90 degrees. At the same time, the coordinates of the test pads storedin the second file or the laser beam deflection coordinate system isalso turned by 90 degrees. Then, the same test is carried out on everypair of test pads stored in the second file.

The slit of the test head is moved step by step at intervals of 1/n (nbeing 1, or 2, or 3, or . . . ) of the minimum pitch P of the pads onthe circuit board.

Alternatively, the coordinates of all pads are checked at first and theslit is moved so that it may not come on top of the test pads.

The two lasers may be two independent pulse lasers, or a pulse laser anda beam splitter that divides a laser beam emitted from the laser intotwo beams. The laser beam may be a pulse laser beam of 1 to 2micrometers in wavelength.

An apparatus for testing a circuit board according to the sixth aspectof the present invention will be explained next in detail with referenceto the drawings.

The first to fifth aspects of the present invention test a circuit boardhaving pads on a main face thereof. Namely, only for pads disposed onone face of A circuit board, do the first to fifth aspects measure acurrent or a voltage in conductive paths and calculate resistance,current, or voltage between test pads, thereby testing the insulationresistance or wiring breakage between the pads.

Some circuit boards have traces on each face thereof. To improveintegration of circuits and workability of wiring connections inassembling circuit boards, more circuit boards have traces on each facethereof. In this case, the first to fifth aspects of the presentinvention are incapable of efficiently and speedily testing the electriccharacteristics between test pads.

Accordingly, the sixth aspect of the present invention provides anapparatus for easily and efficiently testing the electriccharacteristics of a circuit board having traces on each face thereof.

The arrangement of the apparatus of this aspect is basically the same asthat of FIG. 1. To test the circuit board having test pads on each facethereof, the apparatus according to the sixth aspect of the presentinvention arranges a path forming unit and laser plasma switchcontroller on each side of the circuit board.

FIG. 36 is a sectional view showing the apparatus according to the sixthaspect of the present invention. The apparatus has a holder 191 forholding the circuit board, a detector 393 for measuring the electriccharacteristics of the circuit board, and a laser plasma switchcontroller 395. The detector 393 has a first path forming unit 394 and asecond path forming unit 395. The first and second path forming units394 and 395 are arranged along opposite faces of the circuit board 9 andare spaced apart from the faces by a predetermined gap. The first pathforming unit 394 forms a first conductive path 391 between a position S1corresponding to a first test pad i on a trace on a face H1 of thecircuit board 9 and a first power source V1. The second path formingunit 395 forms a second conductive path 392 between a position S2corresponding to a second test pad j on a trace that is the same as ordifferent from the trace containing the first test pad i, on the otherface H2 of the circuit board 9 and a second power source GND whosepotential is lower than that of the first power source V1. The laserplasma switch controller 395 emits a laser beam 10 to a first space P1between the first test pad i and the first conductive path 391 and alaser beam 20 to a second space P2 between the second test pad j and thesecond conductive path 392, to make the first and second spaces P1 andP2 conductive. The detector 396 has an electrical characteristic valuesampler 197 connected to one of the first and second conductive paths391 and 392.

This apparatus employs two test heads 404 and 405. The test head 404 ismade of a glass plate 397 and a transparent conductive film 403 having apower supply pad 401. The test head 405 is made of a glass plate 398 anda transparent conductive film 406 having a detection pad 402. The testheads 404 and 405 are arranged on opposite sides of the circuit board 9such that the conductive films 403 and 406 face and are spaced away fromthe circuit board 9. The power supply pad 401 of the test head 404 isconnected to the voltage source V1, and the detection pad 402 of thetest head 405 is connected to the current detector 197. The conductivefilms 403 and 406 adjacent to the test pads i and j of the circuit board9 respectively serve as the power supply path 391 and detection path392. These paths 391 and 392 are electrically connected to the test padsi and j through laser plasma switches activated by pulse laser beams,and resistance between the power supply pad 401 and the detection pad402 is momentarily measured after the emission of the laser beams.Accordingly, resistance between the test pads i and j is measurable.

This embodiment employs no contact probes, nor vacuum degassing system,nor physical contacts between test pads and test heads (transparentconductive films). This embodiment forms a closed circuit starting fromthe power source V1, passing through the power supply pad 401,conductive path 391, first test pad i (on the reverse side of thecircuit board, for example), second test pad j (on the front side of thecircuit board, for example), conductive path 392, and detection pad 402,and reaching the ground electrode GND. A resistance value is measuredaccording to a voltage applied to the closed circuit and a currentflowing through the closed circuit. The measured resistance value is notaffected by the electrostatic capacitance of the traces.

Measuring time is dependent on a laser plasma switching time, a laserbeam positioning and settling time, and the S/N ratio of a detectedsignal. The laser plasma switching time, i.e., the ON time of a laserplasma switch is 10 microseconds to one millisecond at the maximum. Thelaser beam positioning and settling time is about several hundreds ofmicroseconds. Accordingly, a measurement of resistance between a pair oftest pads will be completed within one to two milliseconds, which isabout 1/200 of the 0.5 seconds required by the conventional contactprobe (flying probe) tester. This embodiment is capable of testingdifferent kinds of circuit boards by changing data for positioning laserbeams without preparing new test jigs. This reduces testing costs andprocesses.

The sixth aspect of the present invention is capable of not onlymeasuring insulation resistance between test pads on traces arranged onopposite faces of the circuit board 9 but also detecting breakagebetween two test pads on a trace arranged through the circuit board 9.

FIG. 37 is a sectional view showing the apparatus according to the sixthaspect of the present invention. The path forming unit 393 for formingthe first and second conductive paths 391 and 392 includes conductivesections made from conductive material similar to those of the fourthand fifth aspects. The conductive material transmits laser beams.

Lasers 1 and 2 and laser plasma switch controllers 410 and 411 of thedetector 396 are arranged on opposite faces of the circuit board 9. Thelaser plasma switch controllers 410 and 411 preferably include laserbeam deflectors 11 and 12.

To carry out a test, the laser plasma switch controllers 410 and 411make the spaces P1 and P2 conductive, and a moment after, the electricalcharacteristic value sampler 197 detects the electrical characteristicvalues of the conductive paths including the conductive sections.

The present invention does not particularly limit the unit for formingthe first and second conductive paths 391 and 392. The paths 391 and 392may be formed of photoconductive material that becomes conductive withapplication of light energy.

The photoconductive material may be that employed by the first to thirdaspects of the present invention.

The arrangement and operation of the apparatus according to the sixthaspect of the present invention will be explained next with reference toFIGS. 36 and 37.

FIG. 36 shows the principle of the apparatus. In the figure, theapparatus measures resistance Rij between test pads i and j that arearranged on opposite faces of the circuit board.

The test heads 404 and 405 are positioned above and below the circuitboard 9 with a gap of several tens of micrometers between them. The testheads 404 and 405 are made of the glass plates 397 and 398 coated withthe transparent conductive films such as ITO films 403 and 406. Theconductive films 403 and 406 are electrically connected to the powersupply and detection pads 401 and 402, respectively. The power supplypad 401 is connected to the voltage source V1, and the detection pad 402is connected to the current detector 197, which is grounded. Any one ofthe pads 401 and 402 may be used as any one of the power supply anddetection pads.

The pulse laser beams 10 and 20 are simultaneously emitted toward thepads i and j, to excite plasma in the space P1 between the conductivefilm 403 and the pad i and in the space P2 between the conductive film406 and the pad j, as indicated with large black dots. As a result, theconductive films momentarily become conductive with the pads i and j, toform an electrical path passing through the power supply pad 401, pads iand J, and detection pad 402. In this state, a current flows from thevoltage source V1, passes through the power supply pad 401, pads i andj, and the detection pad 402, and reaches ground. This current isdependent on the resistance Rij between the pads i and j. The resistanceRij is insulation resistance if the test is an insulation test, andwiring resistance if it is a conduction test. The output current i ismeasured when the pulse laser beams are emitted or a moment after theemission of the beams. Resistance Rmes between the power supply pad 401and the detection pad 402 is obtained by V/i. The insulation resistanceRij between the test pads i and j is obtained by subtracting resistancevalues Rin and Rout of the transparent conductive films and double theON resistance Rlps of the laser plasma switch, from the resistance Rmes.Namely, the resistance Rij is obtained as follows:

    Rij=Rmes-Rin-Rout-2×Rlps

where Rij is the measured resistance value between the test pads i andj, Rmes is the measured resistance value between the power supply pad401 and the detection pad 402, Rin is the resistance value of thetransparent conductive film 403 from the power supply pad 401 to aposition just above the pad i, Rout is the resistance value of thetransparent conductive film 406 from the detection pad 402 to a positionjust below the pad J, and Rlps is the ON resistance of the plasma laserswitch.

In this way, this embodiment positions a circuit board to be testedbetween the test heads, applies a voltage to one of the transparentconductive films of the test heads, and .detects an output current fromthe conductive film of the other test head with use of the laser plasmaswitch technique, to thereby carry out a conduction or insulation testbetween the opposite faces of the circuit board. This embodiment forms aclosed circuit in a noncontact manner, to carry out the conduction andinsulation tests. Accordingly, this embodiment precisely and speedilymeasures resistance between test pads.

The resistance values Rin and Rout are easily obtainable according to anexperimentally obtained resistance value of the transparent conductivefilm per unit length and the length of a path from the power supply pad401 to the detection pad 402 calculated according to the coordinates ofthe test pads. The accuracy of measured resistance value between thetest pads is dependent on fluctuations in the resistance values Rin,Rout, and Rlps. Each of these resistance values is about 10 kilohms, anda fluctuation in them is about 10% thereof, i.e., about one kilohm.Although it is difficult to achieve an accuracy of several tens ofmegohms to several hundreds of megohms, it is possible to carry out aconduction test at an accuracy of about a kilohm. This accuracy is quitesufficient for the insulation test.

FIG. 37 generally shows the apparatus. A frame 460 supports the twolasers 1 and 2 involving laser sources and laser deflectors. A casing461 is arranged between the lasers 1 and 2. In the casing 461, thecircuit board 9 is positioned between the test heads 404 and 405. Thecircuit board 9 is held by spacers 462 and 463 that are held by the testheads 405 and 404 so that a gap of several tens of micrometers ismaintained between the circuit board 9 and the test heads 405 and 404 inthe casing 461. A cover 464 is fixed to the casing 461 under properpressure. In the embodiment, the test head 404 serves as a power supplyhead, and the test head 405 serves as a detection head. The power supplypad 401 of the test head 404 is connected to the voltage source, and thedetection pad 402 of the test head 405 is connected to the currentdetector 197. In response to an instruction from the controller 465, thefirst and second laser beams 10 and 20 are positioned at the test pads iand j. In response to a trigger signal from a timing controller 466, thelaser beam pulse generation controllers 410 and 411 emit the pulse laserbeams 10 and 20. After a predetermined time (for example, 0.5milliseconds), the timing controller 466 provides a sampling strobe tolet an A/D converter 467 convert an output of the current detector 197into digital data. The digital data are fetched by the controller 465,which calculates resistance and determines whether or not conduction orinsulation between the pads is sound. According to the test sequences,the test pad i on one of the faces of the circuit board 9 is tested withrespect to all testable pads on the other face of the circuit board 9.Thereafter, the test pad i is updated, to repeat the test. After allpairs of pads are tested, the test sequences complete.

The laser optical system of this embodiment has no lenses. Depending onthe diameter of a laser probe determined by the size of a test pad,lenses such as a scan lens may be employed.

This embodiment involves no mechanical movements to test pads onopposite faces of a circuit board.

The embodiment completes a test on a pair of pads within about 3milliseconds. When the conduction or insulation test is carried out on2000×2000 pairs of test pads, the tests will be complete within aboutthree hours, which is about 1/160 of the conventional technique.

It is easy to condense a laser probe to 10-odd micrometers. Namely, thespatial resolution of a plasma switch is substantially equal to thelength of a gap between a test pad and a conduction path. Accordingly,the embodiment is capable of carrying out an insulation test on a pad ofseveral tens of micrometers in size that is difficult to test by aconventional multiple-probe tester.

The conventional two- or four-probe tester always requires mechanicaltwo-dimensional movements of a stage, so that it takes several hundredmilliseconds to test a pair of pads. On the other hand, the presentinvention selects test pads by deflecting laser beams, so that it takesseveral milliseconds to test a pair of pads. Accordingly, 2000×2000pairs of test pads on opposite faces of a circuit board will be testedin about 550 hours by the conventional technique, while three hours issufficient for the present invention. Namely, the present inventionreduces the testing time to about 1/160 of the prior art, when the priorart achieves a measuring speed of 0.5 seconds per pair of test pads andthe present invention achieves a measuring speed of 3 milliseconds perpair of test pads.

A modification of the sixth aspect of the present invention provides anapparatus for testing a circuit board having wiring patterns and traceswith pads for insulation between the traces and conduction between thepads on each trace.

The apparatus has two transparent test heads. One face of each of thetest heads is coated with a transparent conductive film provided withone of power supply and detection pads. The test heads are arrangedabove and below the circuit board so that the conductive films faceopposite faces of the circuit board with a predetermined gap betweenthem.

The apparatus has two lasers one for power supply and the other fordetection, having a wavelength to transmit through conductive sectionsof the test heads.

The apparatus also has unit for converging, deflecting, and emitting thelaser beams from the opposite sides of the circuit board toward separatepositions on the test heads. The apparatus also has a unit for measuringresistance between the power supply pad and the detection pad. The powersupply and detection laser beams are positioned at two test pads on thecircuit board through the conductive sections of the test heads. Thelaser beams cause a gas conduction phenomena (laser plasma switches) dueto plasma excited by the laser beams. As a result, the power supply padof one of the test heads is electrically connected to the correspondingtest pad, and the detection pad of the other test head is electricallyconnected to the corresponding test pad. Then a resistance value ismeasured between the power supply pad and the detection pad, todetermine whether or not conduction or insulation between the two testpads is sound.

The unit for measuring the resistance between the power supply pad andthe detection pad includes a unit for applying a constant voltage to thepower supply pad and a detector for detecting an output current flowingfrom the detection pad to ground. Alternatively, it may have a unit forapplying an AC signal (sine wave or pulse wave) to the power supply padand a detector for detecting the amplitude of an output current waveformflowing from the detection pad to ground. The resistance between thepower supply pad and the detection pad is calculated according to aratio between the input voltage or the amplitude of the input voltagewaveform and the output current or the amplitude of the output currentwaveform.

The test pad corresponding to the first test head is tested in relationto all testable pads corresponding to the second test head. Thereafter,another test pad corresponding to the first test head is selected, torepeat the same test. In this way, the conduction or insulation test iscarried out on every pair of test pads on the opposite faces of thecircuit board.

As explained above, the apparatus according to the first aspect of thepresent invention employs a light transmission mask such as a liquidcrystal mask whose patterns are electrically controllable, and two laserbeams, to speedily test conduction and insulation between optional testpads in a noncontact manner. The apparatus according to the secondaspect of the present invention employs a photoconductive sheet on whichtransparent conductive film patterns are entirely formed, to shorten theactual length of a conductive path, reduce the ON resistance of theconductive path, and improve the accuracy of measurement of insulationresistance. The apparatus according to the third aspect of the presentinvention employs no light transmission mask, to shorten the actuallength of a conductive path and accurately and speedily measureinsulation resistance.

Many different embodiments of the present invention may be constructedwithout departing from the spirit and scope of the present invention,and it should be understood that the present invention is not limited tothe specific embodiments described in this specification, except asdefined in the appended claims.

What is claimed is:
 1. An apparatus for testing a circuit board havingwiring patterns and traces with pads, comprising:a means for holdingsaid circuit board; a means for detecting the electrical characteristicsof said circuit board, having a conductive path between a positioncorresponding to a first test pad on one of said traces and a firstpower source as well as a second conductive path between a positioncorresponding to a second test pad on another of said traces and groundpotential; a means for controlling a first laser plasma switch in afirst space between said first test pad and said first conductive pathand a second laser plasma switch in a second space between said secondtest pad and said second conductive path, said first and second laserplasma switches being activated with laser beams to make said first andsecond spaces conductive; an electrical characteristic value samplingmeans included in said electrical characteristics detection means andconnected to one of said first and second conductive paths; and asealing means for sealing a space between a photoconductive sheet orglass plate and said circuit board, and the sealed space is filled witha pressurized or depressurized air or rare gas, wherein said laserplasma switch control means makes said first and second spacesconductive, and a predetermined time thereafter, said electricalcharacteristics detection means drives said sampling means to detect anelectrical characteristic value in said conductive paths.
 2. Anapparatus as claimed in claim 1, wherein said conductive path formingmeans is made from laser transmission material.
 3. An apparatus asclaimed in claim 2, wherein said conductive path forming means is madefrom photoconductive material.
 4. An apparatus as claimed in claim 3,wherein said conductive path forming means is made from photoconductivematerial and has a means for supporting the photoconductive material. 5.An apparatus as claimed in claim 4, wherein said photoconductivematerial support means has a function of controlling light transmission.6. An apparatus as claimed in claim 5, wherein said photoconductivematerial support means is a liquid crystal mask.
 7. An apparatus asclaimed in claim 6, wherein said photoconductive material support meanshas a means for controlling patterns to transmit light.
 8. An apparatusas claimed in claim 1, wherein said laser plasma switch control meanshas a means for emitting laser beams and a means for deflecting thelaser beams.
 9. An apparatus as claimed in claim 3, wherein saidelectrical characteristics detection means has a means for exciting thephotoconductive material.
 10. An apparatus as claimed in claim 1,wherein the electrical characteristics detected by said electricalcharacteristics detection means include resistance, voltages, currents,and insulation resistance.
 11. The apparatus of claim 1, wherein saidrare gas consists of at least one of argon and neon.
 12. The apparatusof claim 1, wherein said sealing means comprises an O-ring.